Diggin the truck btw - nice find and surprised I didn't see it.

As for fun of gas gauges: full tank displayed is zero or near zero resistance in the sender / wiring. Kinda why they never read full but anyway ya didn't mention resistance across the tank senders (90 ohms empty, 0 ohms full). Maybe try it other way: go to sender lead at dash ckt board, look for a ground instead of some resistance, make the ground go away.