My 70 300 had a small meltdown under the dash about 10 years ago, went thru the firewall and out all the way to the alternator. No fire, but burned a lot of insulation on 1 wire and some neighbors. Was able to drive back home and stop it somehow (was 1/2mi from home).
I don't find out exactly where it shorted, but it was not related to the ammeter.
I don't remember all the details, but 2 things stand out in my memory:
The blue ign feed wire that runs thru the dash, out to the Vreg and alternator, has zero fusing on it.
There were a couple of errors in the FSM schematic also, or my car had been wired differently (but all looked very original).