Back Spacing, which way?

Looking for some help from the wheel experts.

I am working with a 1972 Plymouth Scamp. I just installed an “A” body 8 3/4, Doctor Diff large bolt pattern axles, Drum brakes, 15 x 7 Rallye wheels with a 4.25 back space and mounted 255 60 R15 Cooper Cobra GT tires that have a section width of 10.2”

The inside bulge of the tire is 1” away from the leaf springs on both driver and passenger side and the outside bulge of the tire is within an 1/8” of rubbing the lip and would rub the inner fender when hitting bumps in the road and I would rather not cut the lip up to make it not rub. I have looked through the wheel tire combos thread and lots of people use the 4 ¼ backspace but mine are just too close for me.

I work swing shift and spend 3 to 4 hours in the mornings working on the car before work and another couple hours when I get home from work and I’m trying to figure out wheel offset. My brain is foggy and I can argue with myself both ways on offset. One voice says I need a 4” offset and the other voice says I need a 4.5” offset to move the tire inboard more. I have spent some time on google and for the life of me I keep going in a circle.

Wheel Vintiques offers the Rallye wheels in three offsets. A 15x7 wheel in 4 ¼ (which is too close), 4 1/2 and in a 15x8 wheel with an offset of 4 5/8” but do not offer a 4” offset.

Can somebody point me in the right direction that I need to go?