Quick Backspacing Question

I'm planning on buying a set of rims for my 1970 Dodge Dart 340 soon. I've looked around and have seen that people really like to run 15x7 rims with 4.5" of Backspacing. For my front wheels I plan to have a 15x7 with 4" of Backspacing with a 215x60R15 tire. I feel like this should fit fine. If worse comes to worse I have a fender roller that I can use. For the rear I'm planning on a 15x8 rim with 4.5" of Backspacing with a 225/60R15 tire. Has anyone run anything similar to this before? I'm unsure that if since I'm using the 8" rim in the back if I should give it a little less Backspacing seeing how I have a fender roller I could use. Just to be safe and to avoid having the inside of the tires rub.