315/60R15 vs 295/50R15 Rear End Length for Dart Sport

Where the tires sit depends mostly on the back spacing of the wheels. The tires themselves have little to do where they sit other than their width. The 295/50/15 tires are only 10 mm narrower (3/8") but their height is only 26.6" so they should clear the front and rear openings of the fenders fine.

As for the axle flange to axle flange dimension, if it's a stock width A-body rear it should be 52-5/8" axle flange to axle flange