Two pics attached of two slight varied sizes. Obviously I did not have to deal with the wheel opening lip, so I had 3/4" more clearance than someone with a restored car.

275/60x15 on a 15x8 with 4.25" backspace street tire on traditional Cragar SS. 28x9.50x15 on 15x8 with 4.5" backspace drag slick with a Cragar Dragstar.

I don't think you could use a wheel with more than 4.75" of backspace. The wheel well is narrower than the frame rail inside there. Too much backspace and you stuff the tire into the well.