I'm finally finding time to work on this thing again. The holidays kept me busy and broke. Anyways, I have been doing a ton of research on bolting up a chevy LS engine to an AX15. There are a few options, if you go the quick lazy way advance adapters and novak will take you well over a $1000 to get you going. Novak requires you to use a stock bellhousing and their adapter plate $319.00(part# GMAX15) to mate to the ax15. Advance Adapters has a whole bellhousing $483.24(part# 712567V) that mates the 2 together. If you go the Novak route, you need a bellhousing that fits an 11 inch clutch/168 tooth flywheel. LS engines only came with a 168 tooth flywheel, so the more common 10 1/2" bellhousing will NOT work.
There are differences with the 4.8, 5.3, and 6.0 that should be noted. The 4.8 and 5.3 use the same block and 3.78" bore, the 4.8 just simply uses a shorter stroke crank. The 6.0L uses its own block for the 4 inch bore. Unless you plan on a rebuild, beware of early 99 blocks. Gm tried to use the same block for all 3 motors! So the cylinder walls were way too thick for the smaller bore 4.8 and 5.3 causing overheating problems. If you find an early 4.8/5.3 for cheap that needs a rebuild, chances are you could simply bore it to 4." to make a 6.0L.
Any year 4.8/5.3 you can bore it out to 3.898 to make it a 5.7L. So basically an iron LS1. The 99s and 2000s used iron heads instead of aluminum. The crankshaft on the old GEN 1 engines (350s, etc.) stick out .400" further rearward than LS engines. The 99-2000 4.8 and 6.0L when coupled to a manual trans also stuck out .400" further than the 4.8/5.3/6.0 with automatics. Yes, you read that right. They ran different cranks for the autos and manuals in 99 and 2000. Here is a pic of a 99 flywheel (left) and a 2001 flywheel (right) side by side on the crank mounting surface on 2" blocks for comparison. The location of the starter ring shows the .4" difference.