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

The Charging System has nothing to do with the cranking and running, so don't bother going down that road.
Here is a quick-check for a jumped chain.
Find the plugwire that goes to #1 sparkplug. Mark the side of the distributor in a direct line, with the center of the tower. Pop the cap off. Make sure your rotor is fully seated. Turn the crankshaft, until the rotor comes to the mark you made. Remove the #1 sparkplug. See if the piston is close to TDC. If it is not, make it so. After you have done that as close as you can, read the balancer. You are looking for a matching TDC mark on the balancer , that is aligned with the TDC on the timing tab. We are not looking for perfection yet. When you find it, Move the crank to indicate about 10 to 15 degrees advanced.
Now go back to the rotor and see where it is relative to the mark you made;
a) If the rotor is less than 1/4 of the distance between towers, this is a good indication that the chain has not jumped. but
b) If the rotor is more than half-way towards either next closer tower, the engine will not run no matter how ferocious a spark it has.
c) If the rotor is even more than this, farther away, then the chain has jumped
d) if the rotor does not move with the crank, then the chain is likely broken.

If you suspect no spark;
Turn the crank until the points are open. Near ground your coil-wire, .25inch should be good.
Find a small screwdriver with a small shank on it. remove the rotor. Turn the key to run.
Now short the backside of the point arm to the base plate, with a plunge and recoil action. That is to say, your screwdriver will be acting like the points. You should see a tiny spark on the screwdriver tip during the action, and a simultaneous spark at the near-grounded coilwire.
If you do NOT see a spark at the screwdriver, one of five things is wrong;
1) there is no power at the coil negative
2) the point plate is not grounded.
3) the condenser is internally shorting to ground.
4) the screwdriver is non-conductive.
5) the wire from the coil to the points is defective.
Figure out which it is and fix it; easy-peasy.