With an 8.25” rear a 8” rim with 4.5” of backspace will be pretty close to centered. It will be a little closer to the springs, but you need more clearance to the quarters than you do to the springs.
Unless you have a non-A body width rear axle, you won’t be anywhere near centered with an 8” rim and anything less than 4.5” of backspace. If you’ve got a Dart with an A-body 8 3/4 and BBP axles you should end up with about 6” from the wheel mount to the springs, and about 5” from the wheel mount to the quarters. With an 18” rim and typical tire overhangs, that means you want close to 5” of backspace on an 8” rim with the springs in the stock location.
Yeah, I’m gonna go with mrgtx on this one. Just telling someone to “measure” isn’t helpful. A tape measure by itself won’t give you accurate information, and I don’t see you telling anyone how to accurately measure their cars. Or how to then take those measurements and figure out the backspace and tire clearance. It’s true, none of that is all that hard to do. But you aren’t explaining how to do it, so you haven’t helped any.
So, I have to wonder myself why you keep posting when you’re not adding anything useful to the conversation. You obviously don’t seem to need the information that’s being covered, so, don’t read the thread. Pretty simple.