dimed lights and corrosion
Frankly it sounds like you have a multitude of problems and "I could write a book," or more correctly "books have been written."
First the battery itself may have a problem
Next, when the engine is up on RPM, it could be overcharging. Could be simple, need a ground jumper from engine to regulator, or worse
The COMMON problem (Google up "Mad electrical") is that the old bulkhead connectors are JUST not up to the task. So consider performing the popular so called bulkhead/ ammeter bypass. This will remove a lot of the load off the bulkhead connector.
It MIGHT be you have either an older series alternator--no known for low RPM underperformance, OR it just may have a problem, like one phase of the stator damaged or a one or two bad diodes.
Last, it may be time to consider installing headlight relays.
It MAY be time for a new harness