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

I have not seen where you have VERIFIED that the engine is still in time and the timing chain has not jumped. Remove #1 plug. Remove the coil wire. Have someone bump the engine over with your finger covering #1 plug hole until you just feel compression trying to blow it out. Now, look down at the timing tab on the timing cover and find the mark on the balancer. You will need to turn the engine "on around" a little more and line the mark on the balancer up with the zero on the timing tab. Then take the cap off the distributor. If the rotor is not point to #1 plug wire, the timing has jumped.