Back spacing for slotted mag rim wit 235/75/15 8-3/4 rear

So you're saying if you went with 4.5" backspacing you would gain 1/2" between tire and fender and lose 1/2" between tire and leaf spring.

Check measurement on both sides of the car before committing to new rims.

Why not run it as is?

That's a big tire
235 x 0.75 / 25.4 x 2 + 15=28.88"
235 / 25.4 = 9.25"

Tire bulge past rim is 1.125" on each side