any input

Well it is a lot about personal preference. But a 7 3/4 rear is the smallest you can get..so not highly recommended. I would say go try to find a 8 3/4 and that will satisfy your needs, I am sure an 8 1/4 would work just as fine. What is the 340 going to have that you are building?

As far as the motor this is much about personal preference, I personally would say go for a 340, you can either buy schumaker bolt in motor mounts or just buy a v8 k frame for it. For a big block motor you need motor mounts and most likely would use fender well headers (which I personally hate, you have to cut the car up).

As far as the options there should be a fender tag on the drivers side fender with a bunch of numbers and letters, post it here and we will be happy to decode it!