64 rear end help

If your car is just for the street and has 350 or less horsepower, get a '73-6 A-body 8 1/4 rear end, bolt it in, and don't worry about it. If it's going to see the track or has lots of power, get the 8 3/4 and order some big bolt pattern (BBP) axles for it. If it's not an A-body housing, you'll have to have it narrowed if you want any kind of tire clearence.