BackSpace question with a B-Body 8 ¾” in an A-Body Duster

I too am doing the uncut 68-70 b body 8 3/4 in my Demon but a 15" drag wheel instead, same MP offset hangers/shackles

drum to drum the 68-70 B body axle is 3" wider (1.5" a side) and I plan to look at wheels in the 4.5"- 5.5" back space range.. going to run a 275/60-15 drag radial