Homemade Spring Offset kit
I would bet the 275 60s are on a Dart sport or Duster where there is more room than a hardtop Dart. I also think 245-60s are widest sure bet on a hardtop Dart.
Regarding the movement of the front spring hanger - doesn't everybody drill a hole in the frame to allow the welded nut on the hanger to fit inside the frame?