76 plymouth duster ignition troubles

IF you get it to run after releasing the crank position on the key, your "start" hot circuit to the coil / ballast is the issue.

IN other words, it will fire up after you release from start position? The start circuit which boosts voltage, bypasses the ballast for cranking, is the issue.

Get a wiring diagram for that year car, trace the start circuit, test it from beginning to end and find out where it is broken, loose, corroded, or disconnected.

I dunno specifics (many here do) but I believe the start (boost) circuit for the coil routes directly from the ignition key switch thru the firewall. I dont believe it is dependent on the starter relay.
.



.