You can definitely fit a 245/60 in the rear of the Dart. I had that size on the rear of my 70 Dart, not sure what the backspacing was on those rims. They were just "Modern" Wheels bought from Schwabs back in the day.
Currently I have SBP rally's and have a 235/60/14, I could have fit a 245/60, but that would've been close for that rim width.
