What flexplate and spacer do I need to run a TH350 behind a 4.8 LS in a 1972 C10?

You need two pieces to make up for where the LS crank ends: a dished flexplate and a torque converter adapter sleeve.

Most LS engines have a crank flange that sits about 0.400 in shorter than older GM engines relative to the bellhousing, even though the bellhousing bolt pattern is nearly the same. Older transmissions and their converters were built for the old position.

• The dished flexplate bolts to the LS crank and moves the converter mounting surface back to where the TH350 converter expects it.
• The converter sleeve fits over the converter nose so the crank supports it properly.
• The flexplate has to carry the TH350's 10.75 in converter bolt pattern — a dual-pattern or LS-to-TH350 swap flexplate does this.

Speedway used exactly this setup (dished flexplate + converter sleeve) to put a 5.3 and TH350 in its '68 C10 project, and sells it as a kit with hardware. A 4.8 has the same short-flange layout.

Exception: some early Gen III 6.0 truck engines (1999–2000) have a long crank flange and don't use these adapters. More in transmissions for an LS swap.