I am doing the same thing to my '79 10th 4-speed.
Check out
www.5speedtransmissions.com. They have a page that outlines exactly what mods are needed to install a Tremec in your T/A.
I researched the Richmond and Tremec. Bought a Tremec TKO600 from Mike a 5speedtrans.
Hopefully, it'll be in my car in August sometime.