My Right rear wheel rubbing...

It's not that the tires are too wide, it's that the backspace on those rims is totally wrong. Being Centerlines they probably have very little backspace, because with the right backspace you should be able to fit 245/60/15's on a '72 Dart, especially if you already rolled the quarter lip back a bit.

Narrower tires depends on how wide those rims are, if they're 7" wide rims you may not be able to go narrower. For example, a 195/70/15 isn't supposed to be mounted on anything wider than a 6" rim.

So, your options are to buy a new set of springs that will raise the back of the car so the tires don't rub, or buy a set of rims that have the proper backspace for your car. Something in the ballpark of a 15x7 with 4.25" of backspace would be a much better start, although that will depend on what rear axle you've got and what bolt pattern axles and brakes you have.