Intermittent total loss of electrical

Dennis Jackson said
Also, I have not been able to identify the fusible link. It looks like it may have been removed??! I can't find it anywhere in the path where it should be, unless it is covered by tape. Unless this is it? Doesn't seem like it would be because it is hard-wired, not easily removable/replaceable.
View attachment 1715554380

Your photo shows correct wiring for a '67.

The fusible link is in the bulkhead connector.
Maybe to see in this photo.

Original

Dennis Jackson said
The ammeter has already been hardwired through the bulkhead bypassing the connector (done by the previous owner).

Maybe in this process the link was eliminated. When you have a photo we can see how it was done.
When the factory ran wires to the ammeter through a grommet instead of using the multi-connector they had a couple different ways of handling the fusible link(s).

Dennis Jackson said
I want to say I’ve narrowed it down to a loose connector on the ignition switch or the switch itself, but that doesn't fit the symptom.

I agree, but the ignition switches do get worn out internally. I'm just replacing the one on my '67 now but its been on its way out for years. The car has at least 250,000 miles on it so not terribly surprised...
When I check the internal positions for resistance, run had as much 300 ohms depending on how I wiggled the key...

Dennis Jackson said
Based on the wiring diagram, I should be able to remove the connector from the switch and still have headlights, interior lights, etc. But after I re-seated the connector on the ignition switch, I haven’t been able to recreate the issue. The ammeter seems to read differently now too (leaning more toward "C" than in the middle).

I agree.
There's two reasons it might be charging at a higher rate. The first simply is the battery is a little more discharged. The second is that increased resistance to the current makes the voltage regulator sees a lower voltage. When the regulator sees lower voltage, all it knows it is to increase the field current. So if the alternator is producing power now at say 15 Volts when before lets say it was 14.5 Volts, the battery will charge at a higher rate.
OK. Three reasons. if the there is a better connection now between the alternator and the battery, less resistance = high voltage seen be battery = draw more current for charging.