Where is your cabin voltmeter connected? Is it on a circuit controlled by the ignition switch? The problem may not be a ground. Simple test of the ignition switch here:

https://www.ford-trucks.com/forums/1...on-switch.html

Three of the four poles within the ignition switch switch power on/off to various circuits. Make sure to test all three poles, as detailed in the link.

 

Well, at long last I am 99 and 44/100ths % sure I found the problem: the solenoid failed.

Finally, last night the starter would not turn the engine no matter how many times I tried. In testing for voltage drops in the system, I found some minor ones that were eliminated by cleaning the contacts. But still there would randomly be drops along the positive cables run. So today, I swapped out the solenoid from my Galaxie and she cranked right off. I always thought solenoids were either good or bad, but it turns out they can be good, goodish, badish, and bad.

 

When you get a new one DO NOT go cheap as they dont last long at all.
I went with NAPA top of the line and been working for years now no issues.
Also make sure it is grounded to the inner fender good or it may not work.
Dave ----

 

I got a higher dollar solenoid, and bought a NORS from back in the day for a reserve.

Added a jumper from negative terminal to mounting; I don't much care for the sheet metal screws for a ground.