15x8, 3.5" BS fit 73 Duster?
I've got 275x55-17's on 17x8 American Racing wheels with a 4.5" BackSpace, and had to use a 3/16" spacer to clear the spring shackles. Mine miss the fenderwell bu about 1/2", and the spring shackle by 1/4", so you should be fine.