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

Did you index your rotor like I described?
Your ignition system could be 100% working correctly, but if the chain has jumped, it will be impossible to guess where your spark is going, other than to say it is not in the right place at the right time. The method I described only takes a couple of minutes and requires no tools.
If you have to move the distributor to align things, that guarantees something has changed inside the engine, and points straight to an out-of-phase cam.