what size SBP wheels can we fit under a rear 1963 fender?

Looks good. Ya a fresh set of springs would probably help.

Yeah, I agree. Not to hijack the thread, but it's getting a fully rebuilt 8-3/4 and a new set of springs. I expect the rear to come up a bit, those are the original 7-1/4 and springs in there now.

Now back to our regularly scheduled info for the OP!!