Cranks but won't start. Unplug ground at field spade on Alternator and it starts?

The thing is, sometimes these are "self repairing" (maybe temporarily) just by wiggling, disconnecting (examining) and reconnecting corroded terminals.

Once again, what I could SEE in your photo indicates corrosion, and the two most important terminals IN THE ENTIRE HARNESS are the big red and the big black ammeter circuit terminals. I would replace them

Ain't 50-60 year old wiring fun?

My 67 when I got it had bone stock wiring, and there was a 1.5 or more volt drop in the ammeter/ ignition circuit, so the battery was running 15.5 and more