widest tire with spring relocation

If all you want to fit is 275/60s on your 67 Dart, you don't need to do the full 3" spring relocation. If fact, there wouldn't be a point to doing the full 3" relocation if you didn't plan on minitubbing, as once you move the spring inward past about an inch, maybe even less, the stock inner wheel tub becomes the limiting factor, not the spring

You should be able to fit that tire with just a 1/2" offset kit. If you're pinching pennies, you can simply redrill the front and rear spring hanger holes a 1/2" inward and install new spring perches, to accomplish the same thing.