255's are a stretch on a Dart unless you've installed an offset spring kit. Some cars get away with it, some don't. And that's with much shorter 255's.
A 255/70/15 is 29" tall. On my '71 Dart there's only 14.5" from the center of the axle to the front bottom corner of the wheel opening. So, zero clearance, not accounting for movement with suspension travel. You'd have to move the axle back at least a 1/2".