Tire/wheel sizes for 68 dart.
67-9 cars must have more room. My 70 has 15x7's with 4.25 backspace and 255/60's. I can't slide my fingers between the springs and the sidewall and the fender lip is even tighter.
They typically have less, the '70+ cars on average have a bit more room. But realistically they're all very similar, I would say that if you looked at an average of the dimensions on all the 67-76 Darts you'd probably just find that the 67-69 cars are on average at the smaller end of the overall tolerance, while the 70+ cars trend to the larger end of the tolerance, with outliers in both year ranges crossing over.
On a Dart, you can fit a 255 into the wheel well
without 1/2" offset shackles. I would not bet on this, as it's very tight and will depend on your car's body tolerances, ride height, spring rate, and even how you drive the car. People here on this board have definitely done it though.
With offset shackles, a 255 is absolutely possible without cutting anything if you have the backspace right. Even 275's are possible with the offset shackle, but again, I wouldn't do that unless I was prepared to roll the quarter lip. Can it be done without rolling? Probably on some cars. But I wouldn't bet on an individual car I haven't measured being able to do it.