Engine, Trans., and Rear Help

Rear end, do a 66-70 B body 8.75, move perches in 1/2 inch each side and get BBP wheels with correct backspace. Cheaper in the long run than an A body 8.75 and swapping it to BBP, iIf you decide to go that way. Plus, if you go with the later front disc set up, you've got BBP in rear already. Running stock wheels you're stuck with small bolt pattern stuff.

Like rumble said, either a 340 or 360 will work well. 360's are usually a bit less money and capable of good HP. Have an HP level in mind and there are a bunch of different build scenarios to get you there. A 360 with 350HP is a pretty mild build and capable of 12 second time slips in a well thought out car.

Auto trans? Run a 904 and build it with good parts. They are capable of holding 500+ hp and less drag than the big brother 727. Converter is KEY to a good running auto combination.

It will save you time and money to have a goal in mind rather than piecemeal and change your goals. Changing at every turn, you'll buy a bunch of stuff then have to buy other stuff when you change again.