What fits a Barracuda will NOT fit a Dart. Barracuda's have more room than a Dart does, by about an inch. It's all on the quarter side, as the frame rails and axle width is the same. But just like Demons/Dusters/Dart Sports the Barracuda has a wider outer wheel tub. So, not relevant to a Dart Swinger. Even less relevant because the Dart's have such a low wheel opening. You can't hang the tires out past the quarters on a Dart. Well, maybe if you installed a 6" lift kit or something. But not on anything even close to stock ride height. On a Barracuda you can fit 275's without moving the springs at all. The absolute widest tire you can fit on a Dart without moving the springs a 255, and most Dart's can't even manage that without rolling the quarter.
You're moving in the right direction. Do you have BBP axles? Because that changes the backspace you're going to need. I know you said you have an A-body 8 3/4, but factory axles would be SBP.
