When one decides to use an LS based engine (4.8, 5.3, 5.7, 6.0, etc) with a TH350/400 and a "regular" TC (built for Reg SBCs and the TH350/400) you need a spacer that goes into the end of the crank and the hub of the TC.

Hughes Performance HP3795 $43.95+shipping from Summit

Advance Adaptors also sales a kit with this spacer but includes a flexplate and some gaskets (not sure why the gaskets are in there) for $150-ish.

Herron Performance has some of these spacers made from 4140 Chromemoly ready to be shipped for $35.