66 Valiant Electrical Issue?

Current passes through the alt gauge. Once there is a short or overloaded in the system it will melt down at the weakest connections. After you remove the wires from the ALT gauge the short or overload is still there somewhere, just disconnected.
Headlights, wiper motors, and blower motors, can draw much more current than they should due to age and wear. Accumulated load becomes overload.
Consider this... Later models have plastic connectors in the wiper linkage that commonly fail. Owners replace those plastic pieces without considering whay they failed. Its typical for them to fail again in the near future. Aftermarket part quality always gets the blame when the fact is the wiper pivots are so crudded inside that they can hardly be moved by hand. Like they say, "Something's got to give".
How does that apply to yours ?.. 67 and earlier wipers have nearly fail proof linkage connections. If your wiper pivots are worn out that motor works harder.
Same principles apply to blowers. ( I know, you didn't say you were running the blower ).
Worn bearings in it, fan dragging in the housing, restricted air flow, all cause the motor to work harder, draw more current.
Headlights ? I dont know if we can buy new sealed beams today that dont draw more current than OEM.
On a positive note... Once every wire and connection in the car is comparable to battery cables, these other issues should show themselves. LOL
Good luck with it.