effects of relocating leaf springs?

On a '72 Dart I think 275's are going to be as big as you can get with the stock wheel well. I tried test fitting the 275/40/17's I have on my Challenger on the back of my '71 Dart, and found that they won't fit without offset hanger/shackle set up. They have a 5" backspace, which put them into the fender lip, but I only had another 1/4" to 3/8" to the springs. My car does have 11x2.5" rear drums, so the rear drums on my car probably sit about a 1/4 to 3/8" farther out.

I'd say 5 3/8" to 5.5" will be what you need to run for backspace with a stock A body 8 3/4. If you go with Dr Diff's 3/4" offset kit, you should be able to run 5.5" of backspace, which should give you enough clearance on the quarters and the springs. I'd rather be closer to the springs than the quarters, and too much backspace can be corrected with a small spacer if need be.