The truck is running again, YaY.
The problem ended up being the wire from the battery to the starter. It was putting thru 12v when tested, but not enough current.

After pulling the starter again today (the swivel head socket adapter is now my favorite tool) I bench tested it again (properly this time) and the bugger spun right up. So then I bench tested it running thru the starter wire and got nothing. $8 fix instead of $50, so I'm eating Chinese tonight.
I rerouted the starter wire so that it doesn't clamp to the engine, then used that mounting point to turn the old starter wire into a engine ground that now runs directly to the battery along with the frame ground.

Abeemantor, I did try your suggestion before starting to tear everything apart again. Good call, it just happened to be the other 25 year old wire that failed, and now I have a backup ground in case the other engine ground fails.

eb