OK, so cleaning the computer ground got me to 10.5, another volt. I also cleaned the negative battery terminal.

Yes the ground strap to the AC bracket/frame/body has crimped connectors for the bolt connection and I can see where that could have corrosion inside the crimp. I cleaned as well as I could with carb cleaner and wire brush as well as sandpaper for the bolt connection.

I'll check your other suggestions. Even though with a measured volt drop the engine runs fine except for the #4, which today I double confirmed is the dead cylinder. Would I get a full 12v when the engine is running? Don't know how to test as the ground is switching too fast for DVM. So would muriatic acid be OK to clean those crimped connections?

This truck has been in NC for the past 20 years. No real salt/water corrosion caused issues. Whole body has no rust.