[RichardJ]

The starter is cranking the engine over, so it's getting 12 V to the ignition switch. If that circuit gets its 12 V by way of the horn relay as a junction block, wouldn't make any difference. The ignition switch needs to connect that 12 V to the Ignition circuit as well as starter solenoid.

The HEI modules are incredibly reliable. I'd check to see if you are getting the 12 V from that 50 year old ignition switch to the batt terminal on the HEI.

Unlike pertronix modules which have a reputation for intermittent problems, any HEI failure that I have ever heard of, including one of my own, when they die, they die.