I want to run a 255/60/15” tire on the back of my 71 Dart like you. I currently 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.
Check how close you are to the inner side of the wheel tub (sometimes the tub is closer than the spring) but it looks like you just need a 15x8 or 8 1/2 wheel with more backspace. Maybe 5-5 1/2"
Summit's site will let you search wheels by width and backspace.