widest tire with spring relocation

Barracuda's have larger wheel tubs than Darts, so you can't compare to the barracuda body style. You can run 275s on a bone stock barracuda, stock spring locations.

The "3/4" inch Mopar Performance kit isnt really a 3/4" relocation. The front hanger is only a 1/2" relocation, that's all the room there is to the frame. Their shackles are a 3/4" offset, so basically if you put the perches on the axle 3/4" in you're side loading the springs. Not much, but you are. Dr. Diff sells an offset kit that's a 1/2", so that's what a lot of people reference.

Regardless, even the 1/2" spring offset makes the wheel tub the most limiting factor. If you're not going to do a mini-tub, there's no reason to do a 3" relocation. It doesn't buy you any more room than the 1/2" offset. And on most Darts you can fit a 275 with just the 1/2" offset.