You can generalize through all A-body.

The body style/shape dictates the distance to the place the outer edge of the tire will rub.

67-69 Darts are just like your 68. Even 70-72 Darts are the same in the rear. And very close in the front as they take a different wheel lip moldling.

GMachineDart would be a good guy to benchmark from.

On my 68 Dart: 245/60/15 15x7 with 4.25" to 4.5" backspacing depending on small or large bolt pattern is about all the rear will take with the car lowered. My rear wheel lips are not rolled and the body has a tiny be more room to the quarter on one side.

Sure there are people that have got more, but make sure their back ends are not all jacked up.

IMHO, the springs are only about 1/2" past the inner fender. But when the rear end rolls, the top of tire tilts in relative to the body. So you need some room there.

In the front I've barely got 225/60/15 on 15x7 4.25" backspacing rim. I've got 245/60/15 with 15x6.5 rims BUT I moved the bottom front of the fender out.

Might consider some 16x7's 5.0" backspacing with 245/50/16's. Then run 1/2" billet spacers in the rear. Or maybe 17x7's and you probably will have to mess with the bottom front fender lip some. Hopefully GMachineDart has some better insight.