First, welcome to CHR! Glad to have you here and looking forward to learning what caused your fire.

In my experience there's simply not enough combustible material in the distributor to support fire. An explosion like DaveW described, but that's a rarity. Like Ryan, I've had old style coils "explode", pushing the top off and oozing a tar like insulating oil out, which I expect would be combustible, but the "explosion" was coupled with immediate stoppage of the ignition - no more coil. I'd think your fire "looks like" it was the coil & distributor, because once ignited many of the older (and newer imported) plug wires and boots have highly flammable insulation. Assuming your engine is clean and not a "leaker", I'd be looking for a fuel leak, like a sunk float flooding the bowl(s) and overflowing? The other thing I've seen a couple of times is a + battery cable shorting direct to ground, setting that insulation on fire, and then surrounding combustibles.