I have a rebuilt th400 with a stage 2 kit from trans go. I want to add a trans brake . Wondering if anybodys done this and if so what i need to do it. Also can I use my old transgo stuff with the new valvebody?

No. The transbrake is a new valve body, gaskets, program plate etc.

Installing a transbrake also requires stripping the tranny down to do some internal modifications for it to work.