I have a 1974 dodge dart sport with the 8.25 rear. I have always wondered if 15x8 and 4.5" backspace will fit. I have installed the offset mopar shackles and front spring mounts. Also moved perches. I plan for 15x8 rallys just not sure if they fit.
With an 8.25" rear and the stock spring locations if you go for 275's a 15x8 with a 4" backspace usually works better, the 4.5" backspace is typically for an 8 3/4 with BBP axles and the 8.25 is narrower.
Now, with the springs offset a 1/2" the 4.5" backspace will be no problem for you.