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

Sometimes you screw yourself by throwing parts at a problem, and can add problems you did not have

You started out well trying to diagnose the trouble, so a few random comments

1...Now that you have it home, "it might be" the cam drive slipped. If the engine is unknown and worn, that might be. Hoping that you have not moved the distributor, and if you had some idea where the timing was set, check the timing. Be aware, tho, that now that you have diddled the points it might have changed some. Points too far open advance timing, points worn more closed retard the timing. If it's WAY off and you have not twisted the dist, "might be" cam drive
2...When testing spark, it is important to be aware of several things, and to also estimate if you not only HAVE spark, but if it's "enough." So.........

A.....Power from key through the ballast to the coil is ONLY in run. If there is a problem in that circuit, the engine MAY fire during cranking and then die when key is released back to run
B....Power in cranking is supplied by the (usually brown) wire connected to the coil + side of the ballast, which comes from the IGN2 contact on the ignition switch. Just like the start contact, IGN2 is ONLY hot in "start" but it is a separate circuit from start

SO...........If you "check spark" by jumpering the starter relay.......you are using the "run" circuit and NOT the bypass circuit. THIS MEANS that voltage is greatly reduced, because you are powering the coil from the "run" circuit through the ballast, and as the charging system is not working, that voltage is now down to "battery" AND is further loaded down by the starter. You will be lucky to get 5V to the coil if doing so
SO............Try to check spark by using the KEY to crank. "RIG" a SOLID core wire to the coil tower (not radio resistor wire) and wire that to a test spark gap. Lisle makes a decent one that is adjustable, for about 10 bucks. You should get nice snappy blue spark at least 3/8" and more likely 1/2" long

IF YOU HAVE THAT don't waste time screwing with the points, etc.
2...Cap, rotor, wires..........Inspect cap for carbon tracking, dirt, moisture, same for rotor and LOOK FOR rotor "punch through." This is not very common with points. CHECK resistance of the coil wire with a meter. "Rule of thumb" for resistance wire is no more than 1000 ohms per foot, but if good, I've never seen one "that high." Usually much lower.
3...Fuel....Is it possible that somehow someone, either purpose or accident, dumped a bunch of water in the tank? Otherwise, there are procedures in the shop manual to test the pump for pressure and volume IF however the throttle causes the accelerator pump to provide a good stream of fuel down the throat, then it should at least have enough fuel in the carb to fire, cough, "do something."

"My gut." If it just quit AKA "as if the key was shut off" my suspect would be electrical and not fuel. But keep an open mind

Random:

Coils can "just fail" and they can fail/ get weak, and then "heal up" when they cool off. The condenser (in the distributor) can be intermittent as well

Carry an (or 2 or 3) alligator clip lead ---don't be afraid, EG to jumper power direct to coil + to see if you get spark or it will fire

Also, this problem could easily be our old friend ---bad terminals in the bulkhead connector