Troubleshooting 73 Dart Slant 6 not starting.

A quick test is this;
I call it the one-spark test.
1) Near Ground the coil tower, and cycle the key off/on/off.
Every time the key is turned off, you should get exactly one spark. If you don't get it,
2) First check the power supply line, from the ballast resistor for power, and
3) then the case ground back to the battery. If both are good,
4) sub in a different known to be working, ECU, and try again. For this test, You can sub in, ANY Ecu, regardless of application.
5) if you do get the one-spark, but no trail of sparks on cranking, then
Pop the cap and make sure the distributor driveshaft/reluctor is rotating during cranking,
6) that the reluctor gap is anything less than 020. If that is Ok, then
7) check the pick-up for continuity. IDK the spec, but I do know they work just fine at between 160 to 320 ohms, at summer temps, lol..
8) open circuit makes it junk.

If you suspect the coil, again near ground the tower, Run power to the + side and install one end of a long jumper onto the other terminal, all by itself. Then, watching the gap, swipe the other end of the wire on any good ground, looking for a spark. This will only work if you can make and break the ground connection very rapidly. So it may require a little learning curve.
I drag the jumper-wire over an old file, laying on a grounded surface, and watch the lightning show.. You can't hurt the coil during this procedure , so have at it.