do you have a flywheel now? if not, you can go to GM and get a .400 spacer, and a flywheel for a 4l80e trans, the spacer goes bet the crank and flywheel-also get bolts
summit has spacers if using the 5.3 flywheel, which is diff than the 4l80e flywheels
We just finished putting a TH400 behind my ls1. We used a TCI flexplate and a Yank converter built specifically for the TH400/ls1 setup. Like others said you can achieve this combo through various GM flexplates and spacers if you wanted to go that route.