I want to run a 255/60/15” tire on the back of my 71 Dart like you. I have a 235/40-18 back there with a 18x8” wheel with a backspace of 4.5” and like you it’s right up to the fender lip especially on the driver side. But I do have a good inch and a half more space to the springs.
If you don't have to have a 15"wheel, check out late model mustang wheels (maybe torque thrust M for mustang) . They have the 5 on 4.5 bolt pattern and more backspace than the typical musclecar wheels. 17x7 or 8.