Wheel/tire combo for '73 Dodge Dart sport

I think your goals for the car would be the best place to start.

I'll follow up with saying that if you're sticking with 15" wheels, you're going to keep coming up with BFG Radial T/A as one of the few options for you. Fortunately, they're OK (considering that they're All-Seasons), they look good, come in a broad variety of sizes.

They aren't high performance tires by modern standards...not by a long shot.

Folks say that Duster/Dart Sport/Demons can take 275/60/15s with 4.5" backspacing on the stock axle.

I've yet to do this myself as I'm still shopping for a replacement rear axle and I might end up with dimensions other than stock.