1972 dart - stopped running while at 50 mph ? no warning

Condenser.

I think you said you changed points but never the condenser.

If you jump the coil from battery +.. that bypasses the ballast, rules it out and allows you to test for spark. You can do all that and jump it at the starter switch/relay on the fender by the battery with a screwdriver.

Worst case the timing chain jumped.