Tire/wheel sizes for 68 dart.

You might include which axle you are running as that will affect what backspacing you would need. I have a B body axle in my 67 with the offset shackles and about the largest tire I'm comfortable running is a 255 section width. I did have to trim/roll the fender lips on the inside of the wheel opening though. I run some moderately large offset wheels to make it work (they are meant for a front wheel drive car), but they keep the tire pretty well centered in the opening with the axle I have. With a stock axle I imagine you'd be closer to neutral backspacing, but I don't know the numbers off the top of my head.