This is a build log for the truck LS engine installed in my 1949 GMC 100 pickup.

The goal for this LS swap was a stock 300hp in a truck that weighs roughly 3000 lbs. Transmission to be a manual 5 speed. All the factory fuel injection system is used. Differential ratio is 3.54:1 with limited slip.

I wrote this post to document my build, but maybe someone on this forum might find something useful.

This is a photo of the engine installation as it stands today. Anticipating a first start in the next couple of months-




The build started with the purchase of a 5.3L LM4 aluminum truck motor from a 2004 Trailblazer for $650. The engine was in the Trailblazer when I bought it and the odometer read 94K miles. The oil on the dipstick looked like it’d been changed recently and when I removed one of the valve covers, the valve train looked squeaky clean, so I decided I’d take a chance on the engine and run it ‘as-pulled’


I bought the engine as a long block and then found a drive by cable harness and accessories/PCM from a 2001 Suburban with a LM7 5.3L. I’ve done other conversions with the electronic throttle, and I prefer the simplicity of the cable throttle



Although I elected to run truck accessories I used a LS1 intake, if for nothing more than aesthetics. I used an idler pulley relocation kit from ebay to keep the accessory belt from hitting the throttle body. Injectors from an F-body LS1 were used after modifying the harness. I had to use a water pump from a later model Trailblazer that had the water outlet at a different angle to clear the throttle body too.

The oil pan on the LM4 installed in the Trailblazer has a hole through the middle of the pan that a suspension tie-rod passes through. I removed the odd Trailblazer pan and replaced it with a pan from an LM7. The oil dipstick in the Trailblazer was in the pan, not the block, so I had to drill a hole in the block for a dipstick to use the LM4 engine.

The installation of the engine in the 1949 chassis was pretty straightforward. I’ve seen a few LS engine installations and several were pushed back into the firewall leaving enough room for the truck accessories, reasonable air intake and a big puller fan on the radiator. I fab’d the motor mounts with engine motor mount plates I bought off ebay. I used urethane bushings in 1.5” DOM tube as isolators.

The firewall had quite a few holes in it, and my metal fabricator said that a new firewall was about the same labor as patching all the holes in the stock firewall, so we replaced the stock firewall with this one, which allowed the cylinder head to move back about 2" more than the stock firewall did



Here it is spot welded into the cab, mudded, blocked and the first coat of primer



Next installment will be headers and exhaust

 

VERY nice! I'll be watching...

 

Headers and Exhaust

I wanted to use the factory brake and clutch pedals as well as a rear-steer rack and pinion from a Jaguar IFS. Pacesetter Headers is local to me in Phoenix, and the sales manager graciously let me try several sets of LS headers for Camaros, LS trucks and GTOs, but nothing fit. Some universal shorty headers came close, but still would have required three universal joints and a very tight turn at the discharge to clear the rack. I decided rather than trying to make something off-the-shelf fit my truck, I’d try building them myself.

I started with a set of plastic header ‘legos’ from ICEngineworks in Austin, Texas and a set of 0.5” stainless flanges along with some ebay stainless slip-on collectors. Here’s a picture of one of my early designs using the ICE legos



Once I had a general idea of how the headers needed to look to clear all the obsticals on the engine and the chassis rail I built a fixture out of wood, placing the header flange correctly in relation to the collector then building the primary tubes to connect the two




Here’s another photo of the header fixture on my bench. It works for both left and right sides by pining the flanges if different positions. The wood fixture was only used to tack weld the primary tubes. To make the final fit-up match the chassis, the primary tubes were tacked to the flange in the truck



Once the primaries were tacked, fit checked and welded, the flange and collector was moved to the engine to tack the primaries

There was no expense spared in the fixturing of the primaries in the flange - bungee cords, paint sticks – whatever it takes.



Tacked and ready to finish weld



A little detail on the tube fit-ups, The tube in the middle is a mock-up of the steering shaft and u-joint where it passes through the header



Completed headers before coating




A finished header sitting on the floor ready to go in the truck




Finished headers installed on the truck. Even though they’re stainless, I had them ceramic coated to try and keep the heat down in the engine compartment.



All the header fab work was done on the chassis before the front doghouse was fitted. There was an ‘aw ****’ moment when I tried to fit the inner fender wells. The header primary tubes hit the inner fender wells in several locations. My bodyman suggested some additional indentations in the fender well that mimicked the factory indentations. The modification is pretty subtle and now there’s 1” clearance between the header primary and the inner fender



I decided I didn’t want to pay for a complete stainless exhaust system, so I fab’d the rest of the exhaust out of 2,5” diameter steel mandrel bends and painted it with Rustoleum paint that says it is good to 2000F. (yeah right)




With the 2000F? paint



This truck has an IRS, so the exhaust is a lot simpler than going over a live differential. The mufflers are cheap Turbo Thrushes. There’s room for resonators if I have any drone.




There wasn't much room for an x-pipe and I could make a H-pipe out of mandrel scraps. I put a v-band in the middle of the H-pipe to make it easy to get on and off the truck. There’s 3 v-bands and a handful of bolts holding up the exhaust. The exhaust from the headers back can be removed in just a couple of minutes.




For exhaust tips I wanted something simple so I slit a polished mandrel bend from Summit and welded it to the steel exhaust with some 309 rod.



This is a picture of the exhaust installed in the truck - saving my pennies for a bed wood kit.



The tailpipe view

Sorry about that nasty toe

Next installment is the wiring harness

 

Awesome job! and beautiful build.

 

Wiring Harness

The harness for this truck came from a 2001 suburban. I took out all the wires for the automatic transmission added a fuse panel with a relay for the fuel pumps, and shortened or lengthened wires to fit my truck. I followed the wiring advice given on LT1tswap.com to wire the harness



Here’s the harness loomed up on the bench and ready to go back in the truck. All the loom is from Aircraft Spruce- they sell both split and continuous loom, and after having used both I wouldn’t use the continuous loom, just the split. Split loom terminated with silicone tape makes a very presentable harness. The harness extension for the remote mounted coils is shown in this photo too.




All the injector connectors were changed from the truck’s bayonet style to the LS1 style and strain relieved with multiple layers of shrink wrap.





De-pinning and re-pinning the harness is a mind numbing task – one I’d do in the evening while sipping a scotch. To make sure everything was correct and all the grounds were where they were supposed to be, I asked a friend to help me go though the pin-outs and check each connection with a meter.

I mounted the PCM under the dash where it was easily accessible and the OBD port on the bottom of the dash where you’d expect to find it on a modern car. The PCM has a over-center quick release for easy removal. I was still sorting out which wires to keep and the harness rat's nest when this picture was taken.

 

Remote Coils

I’ve never liked the look of the coils on the LS engines, so I remote located them on the front cross-member under the engine. Here is a picture of the brackets I made from aluminum tube, all-thread and some scrap aluminum. I bought two harness extenders, but only ended up needing one.




They are mounted to the cross member that is just behind the balancer.




For wires, I bought a universal 8mm kit at Autozone and cut them to length and terminated them

Next is PCV system

 

Catch Can and PCV

Other LS motors I've built show a tendency to push oil up into the PCV and into the intake tract. I wanted to reduce that possibility with a catch can.

This is the ebay catch can I bought. It's chinesium but all and all nicely done and a bargain at $20. Has an internal baffle and a dipstick.


I first cut the PVC raised bumps off the valve covers. I thought about putting Swageloks on them, but it seemed like overkill and $25 in fittings. My inner-cheapness won out and I drilled out a JIC fitting to accept the valve cover tube and fastened them with JBWeld.

I had to retap all the pipe threads, but now it seals up nice and has a little dipstick so you don’t have to take it off the engine to see if there’s oil in it.

Here is the catch can installed on the motor using a bracket that mounts to the passenger cylinder head




Next Cooling

 

To cool the engine I wanted a modern radiator set up to work with the LS motor. After a couple tries with R&D Specialties radiators (quality was good but didn’t fit the GMC), I gave up and bought a radiator from US Radiator.



This is the condenser dryer from Vintage Air. They clearly don't know the difference between a Chevy and a GMC - their bracket is about 2" short. $375 and I had to remake it. Pissed me off.



I had a 16” SPAL fan left over from another project, but I didn’t want to mount it directly to the radiator, so I built a fiberglass shroud for it that puts the fan about 2” off the face of the radiator and should draw air from over the entire face. The fan is controlled by the PCM via a 30amp relay.

I’ve seen custom speaker enclosures built by stretching flocking material over a wooden form, so I decided I’d try that technique to make a fan shroud.

I started with a piece of particle board cut to the same size as the face of the radiator. To form the edges of the shroud I used 1” aluminum angle that I mitered to fit the particle board base. To mount the fan, I bought a 2” thick Styrofoam disk from JoAnn’s and topped it with a piece of round aluminum that would form the mounting base for the SPAL fan.

Here's just the Styrofoam


Here it is with the aluminum disc to mount the electric fan


I bought flocking material from JoAnn’s but it was so thick it sucked up massive amounts of resin and when it cured the surface was very rough. The first one I made twisted up like a pretzel when I tossed it in the bed of the truck after taking it off of the form and going to lunch. Lesson learned – don’t take the part off the form until the resin has cured for a couple days.

I’d read that cotton tee-shirts worked too, so I dug an old one out of the drawer and used it, and the results were much better. I cut it up, stretched it over the form and stapled it taut on the backside. I gave it a couple of coats of resin, let it cure and then popped it off of the particle board form to glass the back side.


After the first coat of resin


Then I fiberglassed the backside, encapsulating the aluminum parts. I cut the hole with a router and a jig using a HSS router bit



Here's the mock assembly before paint. I'd already started sanding out the resin


The fix for the shitty Vintage Air parts that didn't fit. Bent another bracket and attached the VA bits with aircraft rivits



The radiator hoses are stainless tube with silicone couplers. I'd hoped to polish them, but the mandrel tool that made them must have been worn out - I couldn't get a decent fit-up with egg shaped tube. Gave up and ground the welds and painted it black to look like rubber hose.



The lower return hose ready to TIG



For a recovery tank, I found a one liter aluminum tank on ebay. There wasn’t a convenient place to put it in the engine bay, so I mounted it in the passenger wheel well after welding some flanges onto it.



Next - fuel system