here is what a 30 series would look like on a 71 frame.
6inch driven
View attachment 272593
7inch very tight but it works
View attachment 272594
also note i just made my mounting plate with ear's for the t/c cover. if you decided to use a 7inch you would have to spit the cover open and re fiberglass it in and the bottom would have to be cut open to clear the foot pegs.....lot of extra work make it look good.
if you stay with the 6inch and a 30 you would also want to look into modifying the driver so it does not go into overdrive and allows the engine to rev higher. this is a hot rod build? correct? sounds kind of funny but the way you build your motor might dictate your drive train setup. 20 series does sound better....