The key is on the "other" side of the ammeter.
Like Ralph said, power goes from battery to starter switch to ammeter. Then it goes from the ammeter to the alternator and ignition switch.
You said that only the wire from the starter switch to the ammeter was hot. If the wire from the ammeter to the ignition switch didn't get damaged, it sure sounds like a problem at the ammeter.
I'd suggest putting in a fuse or circuit breaker in that line from the starter switch to the ammeter. I put a 30 amp circuit breaker on my 53 3100. It mounted on the firewall just above the starter. The benefit of the circuit breaker is that it will reset when the short clears. A fuse will have to be replaced.
regards,
Leon
