Max tire size - 68 dart

Don't see why not. The front fender clearance isn't any different than other model year Darts unless maybe you consider the trim studs that MuuMuu is having issues with. That's what I have on my Duster now and it has '71 Dart fenders. You need an 18" rim to clear the tie rods, for a 275/35/18 you need an 18x9" with about 6" to 6.2" of backspace (25 to 27mm offset) assuming you've got 73+ BBP disks. Most likely tubular UCA's as well to clear that deep of a backspace at full steering lock and suspension extension.
Shoot was hoping to stay stock uppers and lowers hmmm. Good to know. I'll stay 255 then go 275 later