Biggest Rear Tire Size

I can tell you that a 275-60-15 on a 15X8" rim with 4.5" will fit. I dont think you will fit a 12.5" section width.

As for the 70 Dart, some guys get a 255-60-15 on the back. Thats pretty tight. I had 225-60-14s on mine and had to jack the car up and hang the rear down low to get them on.