Big Block tranny?

Anyone have any opinions on tranny selections for a mildly built GM big block, figuring 400-450hp or something? I still working on my plan, so don't have the actual motor in hand yet (even though I'm pretty sure on at least the BB part) but it's going in a '50 Pontiac street car.

My goal is something that can handle the power, has enough overdrive not to work the motor, and of course... doesn't break the bank.

Do I have to worry about tailshaft length if I'll need a new shaft also?

What's the main difference between the TH350 and 400 and 700? What about the 4L60E or 4L80E? They don't seem to make them for the Pontiac's...??

:confused:

jb