B body rear in an A body

Yes it will fit,lots have done it. I have two A cars with 68-70 B rears.

The track will widen as the rear is about 3" longer. If you run a reasonable size tire you may not have a problem. You could measure where the tire sits now to the outer well and move it out 1.5" each side to get a rough estimate.

The wheels to sit in the stock location will require more backspace.