This is really sad to see, Paul. To me, it looks like it started under the dash, and progressed from there. My friend almost had the same thing happen to his '62 Dart Maxie clone. He had just purchased the car, and I was helping him with clearing up some mechanical issues and detailing it. We had made new fuel lines, and replumbed them to look more original. We had removed the alternator to build lines at the fuel pump, and reinstalled it. It was getting late in the evening, and I suggested leaving it, and trying to start it the next day, out in the driveway. The next day, we pushed his car out of the garage, to start in the driveway, in case we had some fuel leaks. With my friend cranking the engine over, I was watching for a fuel leak at the pump. I noticed a wisp of smoke at the alternator wire. I looked up to tell my friend to stop cranking, and noticed fire and smoke at the firewall connector. We quickly got the battery disconnected, but the damage was done.
Closer inspection showed that fusible link wires installed by a previous owner at the firewall terminal block had burned off, protecting the underhood wiring. The wiring harness under the dash was cooked, however. FBBO member W. Hudson supplied another harness for us. There was a rats nest of accessory wiring under the dash, that I cleaned up as well. Somehow, this fire started under the dash, and was not connected to anything we were doing with fuel lines. It was an unrelated accident waiting to happen. Luckily it happened in the owner's laneway, close to tools, rather than on the highway.