You may have already done this, but carefully inspect the harness side of the ignition switch and the connectors and terminals. You replaced the switch and it's better but not fixed. That sort of tells me 2 wires in the ignition switch bundle may be touching every now and then. Worth a look. Make sure you have good terminal tension between the spades and the female terminals. They can open up over 50+ years from heat. Report back please.
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