Once you mount a tire, you own it. That is usually in the fine print of any rim sale.
3 3/4" back space is a problem. You can sell and buy new rims, run skinnier tires, trim the fender lip, roll the fender lip, or, get under the car and start tweaking the axle housing position to try and equalize it between the sides. Center the axle, roll the lips, and you might get away with it.
BTW, you don't have new quarters on your car do you? These have notoriously wider lips on the wheel openin compared to stock.