You are going to need a short adapter for the round pattern TH400/NP205, and a short 32 spline NP205 input gear.
The adapter will need to be notched for the shift rail, and at least one of the 205 mounting holes will need to be redrilled.
Check out the Chevy section on Pirate; there's a guy named Grumpy Old Fart who can lead you the right way.