70 Duster - Tire - Wheel - Back Spacing ?

I have seen a pick of a dart sport which is similiar to a 70 duster. It had 4.25 rear backspace rallys with 15x8 size. I once saw a 71 duster on ebay and the backspace was 4 inch even and it looked sweet. Wheel was within wheel well but very close to outside but still not sticking out. I actually asked the seller what backspace since he didn't provide that. So i think i will have some 15x8 rallys made with 4" to get the killet look!!:D