Should work. Based on those measurements I would say you've got an SBP 8 3/4? The 275 is about the biggest tire you can fit on a Demon/Duster/Dart Sport without moving the springs or modifying the quarter lip, so, depending on the body tolerances of your particular car it could be close in some spots. I think with the SBP set up you should be ok with that backspace. You should have between 3/8" and a 1/2" to the springs with that backspace by the math.
It's also about the tallest tire you can fit out back without shimming the hangars back a little to center the wheels in the tubs.
That's really not true. If you've converted to BBP you need more than 4.5" of backspace on a 15x8 if you want to keep the tires inside of the quarters. 5" up to 5.25" is where you need to be if you've gone to BBP axles and brakes. Even on a SBP car 4.5" isn't always best, on a car that's stock ride height or lower it'll be tight to the quarter lips. All you have to do isDuster at that wheel mount to spring measurement. 6". That means a 4.5" backspace is going to leave at least a 1/2" to the springs, but realistically more. And you don't need more, especially if you're trying to avoid cutting the quarter lips. Run that spring spacing down to the minimum, more like 3/8". I think the 4.625 backspace is a better deal than 4.5", worst comes to worst a small spacer fixes too much backspace. Too little and you've got no options.
And, if you have a 7.25 and not an 8 3/4 the measurements are different again.
