During a 4.8-to-6.0 swap, an owner hit a fitment problem at the back of the crank where the flexplate bolts on, and wondered about the crank spacer others mentioned.

This is usually the long-crank 6.0. GM built some 1999–2000 6.0 truck engines with a crankshaft whose rear flange sticks out about 0.400 in farther than a standard LS crank — so parts set up for the 4.8 (a standard short-flange LS) won't sit the same way.

What works with a long-crank 6.0 (Speedway's flexplate guide):

• 4L80E: keep the stock LS flat flexplate. It also supports a TH400 with no other changes.
• TH350: install a TH400 torque converter. It gives the TH350 the 11.5 in bolt pattern that fits the long-crank flat flexplate, with no other transmission changes.
• 700R4 or 4L60E (even an LS 4L60E): no easy fix. The only real option is swapping in a 2001-or-newer 6.0 crankshaft.

How to spot it: on a standard ("short crank") LS the crank flange barely protrudes from the rear seal, while on a long-crank engine it's much more noticeable. Also check the engine's year before buying parts.