Hi. It appears that you have unwittingly wondered into the weird world of electrical idiosyncrasy resulting from bad or faulty grounds. That is a common topic on this forum and discussed regularly.
This is what I did, although I'll admit that I might have gone a bit over-board. I ran a 12awg black wire from the battery terminal and daisy-chained it to most everything of electrical importance under the hood. This included the starter solenoid, alternator, voltage regulator, Duraspark box, plus into the cabin where I used a bolt as a post to connect several extra grounds to instrument panel and radio. I used internal serrated washers or terminals to insure a good electrical connection.
As I understand, if you have any aluminum add-ons (radiator, intake, water pump) I'd run a wire to it as well. Very small electrical resistance can cause current which, over time, can cause corrosion with those items.
Also, I've always been steadfast suggesting that everyone get the Ford Electrical Assembly manual for their year car. It has all of the wires (including colors), connectors and other electrical components of the car, and includes the routing of all the harnesses. Very useful.
Good luck. Keep us informed of your progress.