69 B body 8 3/4 in a 68 A body with 15x7s?

If it does fit, it will be very close. May need to trim the inner lip of the wheel opening on the rear quarter.

I could be wrong, but I believe you will need a wheel with more backspacing if you plan to run a 7" wide wheel and a 255 width tire. That's a lot of tire for that wheel-well, even with a stock width rear-end, and the B-body rear is ~1.5-1.75" wider, per side.