Centering the rear wheels

Hey. This may be a dumb question, but since I just searched through 16 pages on here with no results, I'll ask it anyway. I am going to swap out the 7 1/4 in my 72 dart to an 8 1/4. I know the driveshaft has to be cut down by about 1.5 inches. I had already thought about trying to move the rear axle back about the same distance, so I could run a little larger tire in the wheel well if I wanted. Is there any way to do this SAFELY? I figure you could get a spacer made for the front spring bracket fairly easily. But what about the rear spring mount... AND how would that effect an inboard spring relocation kit? Thanks ya'll.