That is a good point. 15X8 with 4" BS will work with that tire in back. In front, slightly more backspacing will fit the wheel wells better and be less likely to rub the wheel well. They will be more likely to rub the frame and the swaybar, but this is at full lock, instead of "just driving down the road". With too much BS you can get into the Tie Rod and control arm, especially with smaller diameter wheels, so it is a fine line.
I run LS1 and Baer 12" brakes in front on my cars and the ideal BS is much different front to back. In front, the ideal BS is 4.7", in back, the ideal BS is 4". These put the tire in the best location with the least rubbing. In Front, with 12" brakes, 5" BS would probably work best with an 8" wheel, but 18" wheels would be needed due to suspension contact.
