Rear tires after moving springs

275's will fit with the stock spring location and no cutting or trimming whatsoever. Nothing other than proper backspace is needed to run 275's on a Duster, Demon, or Dart Sport. So, the 255's you want to run would have fit easily with no modifications whatsoever.

295 is the widest you can go if you keep the stock wheel tub. That's it, that's all you get. 325's will definitely NOT fit into the stock wheel well, regardless of where you put the springs. Maybe they might work if you put the tail end of your car 4 feet in the air and the tires hang out past the quarters, but that's not the same thing as the tires fitting in the wheel wells.

I have a 1/2" spring offset installed in my '74 Duster. The springs are inside of the inner wall of the wheel tub, meaning, the tires hit the inner wall of the stock wheel tub before they hit the springs even with just a 1/2" offset. I had to trim the lip on the quarter a 1/2" for clearance on the outside too. Now, my car sits low and with SS springs the OP's will not, so the quarter lip probably wouldn't be an issue with 295's and SS springs.

Now, if you do a mini-tub you can pretty much put anything you want back there, 345's would work.