1970 Dart

How you guys doin?
My dart has stock wheel wells, stock leaf spring placement and i just want to know the fattest tire i can squeeze in thier. We had a 15x8 steelie in thier but it only had 4inch backspacing and it wouldnt fit with the 255-60-r15 tire. If anyone out thier has a similiar setup with a different backspacing or tire size and got it to fit can you please post it. any help would be appreciated. (My dad doesnt want me to cut the lip but he said i can trim it just a litle maybe nothing more then a 1/4 inch)