Ditto on nixing the Dexcool....can cause some issues with some metals and can also just turn to gruel over time...
Just use the auto parts stuff...50% will work fine....
I would leave the plain water in for a few weeks to see if anything else crops up....
On the gauge issue...did you wrap the threads of the sender unit if PTFE tape?
If you did it can cause the erratic behavior...use the metal particle sealer available at auto parts stores....
You did install a thermostat in the engine when you put it back together?
Air has a bad habit of getting stuck in the back upper areas of the cylinder heads....your idea of elevating the front of the truck is a great one....I have sometimes installed bleed valves in the rear manifold areas to ensure the air can escape. (Like a radiator drain valve...)
Stay with it young man
