Off the top of my head, the ignition wire at the relay is jut to bypass the ballast resistor when cranking. Unless it shorts out somewhere I don't see it causing a stall situation.
Assuming the problem persists after changing the ECU and coil, I'd be suspect of the pickup coil in the distributor.
Of course 50 year old wires are always suspect.
Some times after the pickup coil gets hot, being inside distributor, it will lose conductivity.
After cools somewhat, reestablishes conduct.
Bad wires, or an internal connection, possible.
Put ohmmeter on plug & wiggle around to see.
Or just replaces with new.
