Back spacing help

I added a 8 3/4 rearend to my abody , I measure from front of drum to side of spring and came up with 7 inches , question is would a 6 inch back spacing rim with tire on work with out rubbing or should I go with 5 1/2 ?

Depends on how much tire bulge. A 17-18" tire generally doesn't have much. A 15" tire can have quite a bit.

Is the rear end a stock A body width?