Will it hit 450 whp
 

408 11 to 11.5 compression. Stock 853 heads. 1 3/4 pacesetter heads. Dumped exhaust. Ls6 intake. Nitrous speced cam. Pushing through a th400 and 9 inch.

450 might be pushing it

Depends on dyno. Motor could use a set of bigger headers for sure tho.

Those heads aren't even adequate for a 346

maybe port your heads and get some 1 7/8 headers

Which would be holding it back more heads headers or intake

I'd say the heads if I had to guess

Get some 243/799 heads and get them CNC ported. THEN you will have some REAL heads compared to now.

Best budget heads are used LS3 heads.

^^^^ I agree. Plus, you can find a LS3 intake for the same price as a LS6 with rails, injectors, and all. Sell your heads and intake and it will not take much extra money of yours.

YOUR 853 HEADS FLOW:
flow intake exhaust
.200 128 101
.300 285 135
.400 226 165
.450 244 174
.500 256 181
.550 262 187
.600 262 192
.650 265 195
.700 265 196
.750 250 196

SO,
HP = 0.25714 × CFM × No. of Cylinders

You could technically support 500 hp at crank with the right set-up.

But I would get some ported 243s or TFS225 heads (nitrous exhaust port speced), Dorman LS6 intake (ported), and get a cam spec from a pro, plus if your gonna squeeze it get bigger headers.

Ported 243s from TEA flow 315 to 320, will support up to 650hp.

ls3 heads will support over 650hp, but air speed will suffer due to large ports and i doubt your engine would require over 330cfm, i wouldn't sacrifice air speed for a street/strip car, full race big bore / high rpm / high stall / gears / long duration - high lift cam, I would look into ls3.

What? Airspeed is one of the most important factors for making power. Go ask any induction expert.

I think Big Hammer is saying that there will be enough airspeed with that combo

Turn on a garden hose and the water will dribble out a couple of feet. Now put a nozzle on the hose and the water will spray across your backyard. The water pressure and volume haven’t changed, but the velocity has increased dramatically. Now think about the air and fuel going into your engine’s cylinders. Which would you prefer: slow and lazy or fast and responsive?

http://darinmorgan.net/frank-talk-about-flow-numbers/BORE SIZE

Bore diameter has a great deal to do with the performance of the cylinder head. Not only how much it flows, but more importantly, how it flows. Most modern performance heads used in Drag Racing are designed around the largest bore possible. For a Big Block Chevy that would be a 4.625 bore. The reason for this is simple: with larger bores we can increase valve diameter, valve efficiency and produce more horsepower at the same or higher engine speeds. A big problem arises when people attempt to use these heads on much smaller bores. If a cylinder head design was optimized around a 4.625 bore and someone uses it on a 4.500 bore it shrouds the valves, decreases air flow, and consequently destroys the port’s air speed characteristics. This totally destroys the entire power band from start to finish! You must utilize a head designed for the bore you’re using with smaller ports and valves. Yes, the head will flow less, but it will make more power and accelerate down the race track much faster!

nope.

Bang for buck, slap an LS3 top end on that motor. Like said above sell your stuff and you'll cover almost all the cost of the LS3 setup, and make a lot more power. A 408 is plenty of motor to utilize big LS3 ports, and it sounds like you're setting up more of a drag car than a street car, so you'll be at upper RPM's a lot, the LS3 combo is what I would do for sure.

The final test run was to combine the rectangular-port cam with the Mast LS3 heads. The combination produced the highest peak power numbers of the day (640 hp and 680 lb-ft), but only by 1 hp and 2 lb-ft over the cathedral-port head and rectangular-port cam combo. The LS3 heads did respond better to the installation of the rectangular-port cam. The cam improved power from 5,300 rpm on up, with only a slight loss in power below 3,300 rpm. Obviously there is something to increasing efficiency of the exhaust port with increased duration. Though we were happy with the results of the cam test, we were still surprised that the LS3 heads offered little or no power over their cathedral-port counterparts. Given the disparity in airflow, we expected to see 10-15 hp, but the results of this head-to-head shootout just go to show that airflow isn't everything. It also showed that you can't go wrong with either set of heads on your performance LS combination.

The results of the head-to-head shootout indicate that (despite the flow differences) the cathedral-port heads were the equal of the rectangular-port LS3 heads. It is interesting to note that the cathedral-port heads offered better low-speed power and produced peak torque lower than the LS3 heads (irrespective of cam choice). This might be attributed to the difference in port volume, though these Mast LS3 heads featured intake port volumes of 256 cc compared to 245 cc for the cathedral-port heads. What the LS3 heads really needed was a cam that could take full advantage of the head flow (372 cfm) offered at .700-inch lift. The use of our .624-lift cam negated some of the additional flow offered by the LS3 heads, though 360 cfm at .600 lift was still a sizable jump over the 335 cfm offered by the cathedral-port heads.

http://www.superchevy.com/how-to/ght...-head-to-head/

That's great but hardly relative to this thread. That test used 245 cc cathedral ports. He's considering using a far far far different cathedral head, and some of us are recommending him go with an LS3 top end and have little or no money invested in the swap.

Going to 245 cathedrals would add about 3k to all of this easy. And then he could make a little more low end, on a car that will have a turbo 400, a 9 inch and a nitrous cam....