Help! How much space gained by rolling fender lips on '70 Dart?

Look, the quarter lip on a 70+ Dart is only about 3/4 of an inch wide. But, as was already mentioned, the wheel tub is not flush with the inside of the quarter. Not only that, but the wheel tub tapers in at an angle. And THEN, about an inch or so above the top of the quarter lip, there's a "bump" in the wheel tub that limits clearance again.

So, how much room can you gain by rolling the lip? Maybe a 1/2". Maybe, and that's the absolute most you'd get. But that 1/2" toward the quarter that you gain only goes up another inch or two, before you have even LESS room than what you had to the original quarter lip. So you just rub on the wheel tub instead of the quarter lip. Maybe it works when you're not moving, but if hit a bump or throw some people in the backseat you'll still rub.

You can't really make room for wheels that don't fit by rolling the quarter lip. If you have a slight rub when you have people in the back seat, or when you're hauling a bunch stuff in the trunk, or when cornering hard etc, then rolling the lip back a little bit can help your cause and buy you the little bit of extra space you need.

Bottom line is, a 15x8 with 4.5" of backspace isn't enough backspace for 255's. You say you have a quarter inch without the drums, so that means you actually have almost zero clearance with everything assembled. You need about a 1/2" to the outside to keep from rubbing, so what you really need is a 15x8 with 5" of backspace, minimum. Because if you have a little too much backspace you can add a small spacer. Too little and you have to cut things, as you're finding out.