Why not go to a Mucie or T-10 4 speed, or maybe a Tremec with 5 or 6 gears??? If you're going to make it a driver, might want to go to an overdrive type of trans. You can run 4:11 or so rear end ratio, then with an od in 5th gear can cruise down the highway at a reasonable rpm....

Might want to include a front disc brake conversion, too. Got to be able to haul the beast down safely, too.

Find a buildable 350, get a 383 stroker kit and some good heads. Lots of low rpm torque works great on the street.

As for the differential, use a 9" Ford or a 12 bolt chevy, they are reasonable priced and will stand up to reasonable horsepower applications without a lot of high dollar mods.