theres a 71 Dart on ebay with 275-60s, stock wheel wells, Add says the springs are moved in and the rear has been shortened. Offset hangers should be enough to move the springs out of the way to use all the stock wheel well. With the springs in the frame and a widened tub a 325-50-15 should fit nice.
These are 245/60-15's on my sons 72 Dart. No tubs or springs moved. Everything is in the stock location and the 8-3/4 is out of a 67 Cuda. Thes tires are on a 15 x 6.5 wheel with a 4.25 backspace. It has a small amout of room to go a tad bigger like maybe 255 or 265 but thats probably going to rub. Its a street car and the 245/60-15's never rub. Ron