USCT inboard spring kit

If you don’t do the mini tub the 3” relocation doesn’t provide any more room than the 1/2” offset, the factory tubs will be the limiting factor with either one.

On a 70/71 dart, the factory tubs are only good for a 275 at the most, so 275/60/15 is probably the max for height and width. Even then you might still need some quarter lip rolling/trimming.
I couldn't get a 275 60 under my 65 Valiant without the 1/2 relocating kit. Once I did that though, I could fit 28/9/15s on 15x9 Center Lines. That was with the stock tubs. I agree 100% though. It's kinda senseless not to tub one when you move the springs inboard. If I was doing more of a "drag themed" car, I'd move the springs inboard on Vixen.