When using a conventional style trans behind an LSx engine you need the spacer that goes between the crankshaft an flexplate. The spacer is there for the converter to index into since the crankshaft doesn't protrude as far back relative to the bellhousing mating flange on the LS engines.
You also need the flat flywheel instead of the bell shaped one.
About $200 worth of parts from GM, because the bolts are also longer and shanked to center the spacer so it all stays concentric.
GM used these parts on the vehicles that used 4L80E's behind the LSx engines.
One thing to keep in mind, 1999-2000 (maybe 2001?) 6.0L had a different crankshaft that protruded further so you wouldn't need the spacer.
Anytime you use a PowerGlide, TH350, TH400, 200-4R, 700-R4, or 4L80E behind these powerplants you need the spacer, flat flexplate, and bolts.