On "some" tractors (or farmer Bob may have modified the tractor over the years) (not sure of your Ford) the big ungrounded battery cable attaches to the INPUT (NOT to starter) side of a starter switch or solenoid etc and at the same place a wire may be attached that provides battery power up to ignition and light switches or alternator (if no ammeter) etc.
I would remove, clean and wire brush, reattach and tighten each and every starter and starter switch/solenoid connection,,,,,,,,,,,,,Alternators output stud and where its cable attaches to any ammeter or battery or battery side of switch/solenoid,,,,,,,,,,Ground cables,,,,,,,,,,,,,,Battery posts,,,,,,,,,,,,Any wire from battery or switch/solenoids input up to dash (switches etc) and any Neutral or push down starter switches like some Fords have.
I'm NOT a Ford man so this may or may not be correct, trolls or correct police don't have a calf now lol BUT I SUSPECT A LOOSE OR BURNED OR CARBONED UP CONNECTION even if on a Ford.
John T