Mini tub with spring location on 70 duster

ok guys I need help. I have a 70 duster stock a body 8 3/4 rear end width. The springs have been relocated to inside frame rails with the mopar minitub kit. My measurments come up with a 6 1/2 in backspacing on a 15x10 wheel. Is that correct and or what is the widest wheel tire combo i can get in there and what backspacing would i need.

Thanks.
Ron