If you speak to a tech rep at Mosier- tell them your measurements. Yes for that much money you should get it right. The a-body 7-1/4 rear will interchange with a a-body 8-3/4 rear. If your wheels fit, just tell them you have a stock a-body and are looking for a stock length A-body rear. I would also verify that the wheel face to wheel face measurement is the same so the wheels do not move in location and hit a wheel well lip.
Personally a used a-body 8-3/4 can be had for less money unless you need something specific or want a certain look.
Good luck in your quest!
Joe
