Time for new tires and rims for the 68 Dart

I haven't done enough research yet, but if you are looking only at one piece torque thrusts, try shopping the two piece. There should be more selection of b.s. And as said above, there are lots of tt copies out there that may have different b.s. available. Torque thrust m, for mustangs have lots of backspace, but I don't know if 15s are available.
You do know that ARE has a custom shop that can make you anything you want, within reason. I had some 15x12s made a few years ago, with minimum backspace. Weren't cheap then, won't be now.
As said above, I much prefer too much backspace versus too little. Too little, the tire crashes into the fender, too much bs, it rubs on the spring, but that can be fixed perfectly with a spacer.