Transmission Options

Well, a few weeks ago, my 74 Duster (slant 6, A904, 7.25 rear) broke its flexplate and damaged the torque converter.
Since i've begun tear down to a shell to do a basic resto-mod, I've also decided to part ways with the 3 speed auto.
I want to get a 4 speed overdrive or 5 speed standard.
Plans are to use it in road course and autocross.
So although i'm not looking to buy right now, i want to compile options and parts lists.
Any info and help is appreciated