I have a 05 4.8 that I emoved from my 05 1500 that had the 4l60E in it about 3 years ago. I bought a 04 6.0 and pulled my 4.8 and installed the 6.0 with my factory 4l60E I had and can't remeber if I had to switch flex plates for sure.
But know I am trying to install this 4.8 in my 55 belair I am using a 4l60E from a 98 s-10. I am trying to figure out for sure what is going to be needed I have a smooth flex plate on the motor with no recessed bolt holes but I bolted the trans up last night to start making mounts and test fitting things. I am aware I will have to enlarge the flexplate holes inwards a tad to bolt this up. But am looking for the infor on a spacer or what is needed for this also which starter will I have to use 4.8/5.3 or 6.0.

 

Guys with all the knowledge around here thought I would of gotten some info on this.

 

I can tell you what I did - hope it applies somehow. IIRC all the LS starters are the same, I wondered about this too, because people were talking about putting a spacer on the crank and THEN putting the flexplate on. I have a 2003 LQ9 from an Escalade. It has the curved flexplate and is sort of recessed in at the center (toward the crank). I have a 4L60 from an old Caprice and needed the spacer and longer bolts which I got from GMPP Parts Direct. First I rounded out the three holes (a lot more than 1/8 inch like some people said) put my curved flexplate on, THEN the spacer and new bolts.

I've been running for 5 months now - no issues.

 

if your using the 4.8 with the 4.8 flex plate and the 4l60 out of the s10 then im pretty sure you should be just using a starter for a 4.8 engine with no spacers on anything....

 

This has been covered a bunch but there isn't one source for the information and much of it is scattered or unclear. I remember contributing to a topic on this very subject and can't remember where it is and it never got added to the stickies.

There are two basic things to worry about in the automatic LSx world, the flexplate and the transmission. Everything else (starter, engine size, etc.) doesn't really matter too much.

The engine needs the correct starter and flywheel / flexplate which means Gen III / IV (LSx) or Gen I / II (SBC). These are NOT interchangeable across platforms (LSx to SBC) but are between engines. There is only one size flexplate for LSx engines even though the starters vary (even by PN and application) the differences are mainly cosmetic. Some truck starters are said to be smaller (shorter) than F-Body variants and although I haven't measured them just looking at the two this appears true (I have both in use).

The differences between the "flat" and "dished" flexplates don't matter as long as you have the correct one for your crankshaft and transmission. The ring gear is in the same place on both when properly installed so the starters will line up without issue. You also can't put the dished one on wrong and get it to work so no worries there. The crankshaft on the early (1999+ ish, usually iron heads) 6.0 engines was .400" longer than all the other (LSx) engines. This is the same as the old SBC / V6 spacing AND the 4L80E. All of this is easy to spot if you pay attention.

In short:
For a non-LSx transmission setup (TH350 / 400, 700, 200 and 4L80E):

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LS transmission setup (4L60E, 4L65E, 4l70E):

• Keep everything that came with the combo (I.E. dished flexplate on short crank).

Transmission Spotting:
For transmission spotting it gets a little tricky but once you know the details it's just as simple. The easiest way to spot an LSx transmission is the top of the bellhousing. If there is a hole for a bolt, it's an LSx, if not, SBC or V6. Also make sure the transmission says "PWM" on the front pump or the LSx PCM won't be able to control the lock-up torque converter properly.

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Sorry for the long post, hopefully that covers it. Any questions, just ask.