When troubleshooting a situation like this, measure the voltage drops across connections not to ground. For example measure from the alternator terminal to the battery.

I would get the factory schematic and start measuring voltage drops starting at the alternator to first point past that, then next point, etc. This will give you some clarity.

Does certainly appear that you have a bad connection. If you have the factory bulkhead that is probably where it is. In my cars that I don't care 100% about originality, I take out the bulkhead connectors for the 2 high current feeds, drill them out slightly and run the wire through the connector eliminating the connector. Works great but you are cutting the wires if you need to take it apart. You can also successfully replace the connectors.