It might be worth checking the thermostat again. I had one in my Pontiac, that after sitting for six months, failed to open. Which is strange, as I thought they were designed to fail open, not closed.
I would stick a thermometer in to see if the gauge is accurate.
If it is too hot, then I'd fire it up after removing the thermostat. With the rad cap off, you should be able to see the coolant coming into the rad. Also, the truck should run cold with the thermostat removed. You should be able to see this with the thermometer as well.
