largest rear tire without relocating springs or?

Dart's only have about 11" from the springs to the quarter lip with everything stock, give or take the production body tolerances. That's from my 71's, the '67-'69 cars can be a little tighter than that. That only leaves about 10" for tire, but because of the low wheel opening on the dart's that width has to be the section width, not the tread width. Even SS springs don't lift the wheel opening above the tops of the tire on a '67+ Dart. The section width on a 245/60/15 is 9.8", section width on a 255/60/15 is 10.2". Really you need a minimum of 3/8" to the springs and 5/8" to the quarter to keep from rubbing on a street car, so the 255's won't work on most Dart's unless you roll the quarter lips or do the 1/2" offset.