Flickering Lights

Brought wife home Monday after successful surgery to remove kidney. Now I am chief cook and bottle washer!!
Great ! thumbs_up-gif.gif

I think drawing out your own wire schematics and cable diagrams will go a long way.
Begining with the main power feeds first as you are doing, seems like a good approach. After that you can adress the modifications for the voltage regulator and the electronic ignition.

The factory scheme uses a 16 ga fusible link to protect the 12 ga wires from a battery short.
This includes the alternator output circuit.
The ammeter was placed to show battery discharging and charging.
upload_2022-2-24_12-0-16.png
A1 was oriignally black 10 ga wire. Around 1966 it more often was red. As long as its 10 gage and you know what it is I think either color is fine.
The fusible link should be 16 ga. That's important. Big enough to handle starting and emergencies when the alternator fails. Small enough to melt before the 12 gage wires.

On your car, someone has connected the alternator output to the battery circuit at the solenoid.
upload_2022-2-24_12-19-18.png

It may have been due to a loose or damaged terminal ?
They may have also rerouted the power feed around the ammeter.
Sometimes the ammeter gets converted to a voltmeter.
It will be importqant to figure out what was done.