After a fire.....

64longroof said
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.

Your analysis seems sound.

64longroof said
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.

Same.

64longroof said
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?

I'm going to guess it was a short to ground. It could have been in the engine compartment or under the dash.
The reasons are:
Key off leaves the main circuit and the feeds from it hot. Items after the fuse box and the headlight switch are protected.
There was open flame on both sides of the firewall. While there is some chance the dielectic grease carried fire through the bulkhead it doesn't look like that.
In the photo of the starter relay and bulkhead, the battery feed shows signs high heat. Those two wires are the lowest.

On the engine side of the firewall check the alternator for internal grounding. Output terminal (Batt) to housing should be open. Then check the output wire to engine and body ground.
The only other wire that could would be hot in the engine compartment with key off might be the feed to the horn relay. In 67 its off of the alternator Batt terminal.

if its not in those, then it prob was a main circuit wire inside.