The numbers on the Cragar box are --- #61815 -6 STD. In doing the math, my son and I came up with 4 1/4 backspacing.
The tires are pretty close to the inner fenders above the wheel lips. The wheel lips on my car were turned up a long time ago. I don't think the 275's will rub, but if I was going to put these on the D/Dart, I would go with a 255/60R15. The 275's are about 10" of tread width and 11" at the sidewalls. The SS springs help a lot on the clearance. This is the car with the vintage Radirs that came on the car when I bought it.
On my '64 Belvedere, the car that the Cragars actually go on, one side is really close to the inner fender. Again, a 255 might be a better fit on this car also. Or it needs SS springs also.