It sounds like your gauge has been broken. Please let me try to explain as you only have one option if that happened:

When the truck was designed and built, the temperature gauge was controlled by a heat sensor and a capillary tube. The tube connects in a sealed system to a bourdon curl inside the gauge. This curls or uncurls based on the pressure of the expanding liquid in the capillary tube (which is what causes the needle to move) and if exposed to very high temperatures can become stuck at the upper most position of the needle. It isn't common, but I've seen it happen and in basic terms the bourdon tube/curl becomes unusable after that. To remedy, you need to buy a new heat sensor and gauge...but not the entire gauge cluster. It will come with a new capillary tube which can be installed very carefully (so as not to crimp the tube) and that should restore your temperature gauge.

If you don't do any of your own repair, hopefully you know a friend who might be able to remove and replace the gauge. Good luck!

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end