65 Barracuda, best width and backspacing?

I have BFG T/A G60/14 bias on 7" at the rear, with 8-3/4" tread. I put these on 30+ years ago. Wheels put on 40 yrs. ago when installed 8-3/4 rear. Problem is replacing with comparable 15" radial with same tread width. Plan is Moser axles with BBP.

Rear Tires.JPG