Joined: 30 Apr 1999
Location: 2000 Silverado Z71 4.9L 4L80E, 2003 Lincoln LS 3.9L V8 5 speed auto
Externally there's no real way to know if you are getting a 5.3 vs a 4.8. The only difference is the pistons, connecting rods and crank. Everything else is the same. IN fact, the blocks are cast (4.8/5.3). The only real way to know would be to pull the pan and measure the connecting rods. You could also try looking in a spark plug hole to see if you can see the top of the pistons as the 4.8 has flat-tops and the 5.3 has dished pistons. They did this to allow the same head to be used on either engine to get the correct compression as a cost saving measure. This way, they only had to make one kind of head.
They make an adapter ring for the trans, or you can get a conversion converter. Otherwise it'll bolt right up, and the Gen3 computer will connect to and run the 4LPOS60E. If this helps, I installed a GMT400 4L80E transmission in my GMT800 truck. I used a converter that allowed one to bolt to the other and had to use a 6.0 flexplate. You won't have to worry about getting a 6.0 flexplate for a 4L60E, the only reason I had to was the bolt pattern on a 4L60E is smaller than a 4L80E. I'd give Finish Line Transmissions a call and ask them how they'd recommend doing it. They'll probably recommend doing it with a converter and they will be able to sell you the correct one. Great bunch of guys up there.
So far as the 5.7 vs 5.3, I recently got into it with a Vortec reg cab truck and ran away from him. Mine's a 2001 2WD reg cab 4.8 that's stock other than dual exhaust (baby turbos, no cats) and an EFI Live tune. All I know about the other guy's truck was he was running Flowmaster 40s, because when he nailed his I could no longer hear mine.