here are the facts about fitting an old style trans to the gen3 engines
1. the gm gen1,2 trans will bolt to the gen3 engines but you only use 5 bolts, the gen 3 engines do not use the middle bellhousing bolt on the pass side( no big deal)
2. you do not need a special flexplate, the curved 4.8,5.3,LS1- just need the t/c bolt holes ovaled out a bit and the 5.3 flex plates have a set of holes that match the small gm t/c bolt pattern perfect
- the 6.0 flat flex plate has the same t/c holes as the large gm pattern(th400 and most aftermarket th350 converters that have a dual t/c bolt pattern on them)
3. you need to use a crankshaft spacer when using a gen1,2 trans and converter there are 2 types:
- hughes gen3 crankshaft spacer part # hup-3795 $44 from summit/jegs, this spacer must be used with the curved 4.8,5.3,LS1 flexplates
- gm stock 6.0 crankshaft spacer 12563532 and longer bolts 12563533, this spacer can be used with either the flat or curved flexplate ( if using the curved style flexplate the flexplate goes on the crank first then the spacer goes on then the bolts, if using the flat 6.0 flexplate the spacer goes on the crankshaft first then the flexplate then the bolts)
So, in the pic above, thats correct for an LS1 or 5.3. And then it appears it'd go the other way for a 6.0, which was the factory configuration. Really depends on what flexplate setup she has.