Will 8-3/4 alxes fit in the old 65 7-1/4 rear?

Depending on what your plans are for the car the 7 1/4 may be just fine until you locate an 8 1/4 or an 8 3/4. It was the stock rear end behind \6's and non performance 273's and 318's, in those applications it provided 100's of thousands of trouble free miles. If your immendiate goal is the bigger bolt circle any 73 and up A-body with disk brakes or a 318 will likely have a big bolt circle 7 1/4, much, much, much easier to find than an 8 3/4 (I gave one away a couple of years ago).

You will just need to remember that you can't be beating on it or it will fail.