I'm assuming your car is an Abody since you couldn't find an 8.75" rear.
If you need 3.5" backspacing, your gonna need a custom rim I'd bet. Buy why don't just go wider and take advantage of that narrower rear? You can move your springs 1" inward each side with a Dr Diff kit for $150, (probably should have thought of that before you welded on the new perches...) and use a wider rim, or you can just get your rims widened, there's several places that do that.
Good luck