Are you getting a custom conversion converter made to adapt the TH400 transmission to the LS1 flywheel setup? If not, you'll need the flat flexplate from the 99-2000 6L, spacer and bolts. They're available all over the place, I got mine from crateenginedepot.com. You'll have to slot the holes in the flexplate to match your converter bolt pattern.

Wiring harness, NelsonPerformance.com makes a nice LS1 conversion harness.