After a fire.....

So review what was going on when the fire started. What was the scenario? That will be one set of clues.
Heat is created when current runs through resistance. More current or more resistance will generate more heat.
Look at the wires connecting to the different components, especially at the connections. Most of the ammeters consist of a metal bar with two studs pressed in, and an insulator. You'll see if there was heat in it, or outside of it, or not even close.

FIRST: Ign was off. Car was parked. I’ve tested ign switch, post fire, and it does not
pass current when ign is in off position.
SECOND: Bulkhed disconnect badly fried on both sides of firewall.
THIRD: Wiring running to alt. and down to starter are fried, although the closer you get to alt, the less so.

Alll orig. gauges. The one change was to put in a 5V transistor/capacitor setup. BUT this was only hot when ign was on. It was lower than where most burning was, and the wires are not melted.

There's a couple ways an electrical fire can start. One would be from normal use of a circuit with a poor connection that got so hot it ignited something flammable nearby.

I've worked on electrical things long enough to know about shorts and corrosion causing heat build up. I sprayed electronics cleaner on the terminals in the bulkhead disconnect, scrubbed w/ pipe cleaners as best as could , no green corrosion, but they didn't look shiny clean, either. No melting evident in disconnect to suggest a previous heat buildup.

The other is from a short in a circuit. A short is an additional or easier path to ground.

I really doubt there was a hot wire short, unless somehow that 5 V setup was wired in wrong? It was making the gauges work, so I kind of doubt this was the case.
Highly doubt the hot wire was shorted under dash. But maybe???? Ivhad been taking that inst. cluster in and out several times, so who knows?

If there was any wire that insulation had rubbed off and was touching other wires, or worse, ground, that would cause a short.

A fusible link, which a '64 probably had, helps protect against a direct short from battery to ground in the main circuit.

I know about those obvious fusible links in the later harnesses, and noticed my car didn't have one.