Somethings to check that cause excessive heat in the clutch bell. Too loose of a slipper clutch, worn out clutch shoes, worn out clutch bell housing bearings, and binding between the spur and clutch bell. If you still can't figure it out try the following: reversing the rotation of your clutch shoe(the clutch bell rotates against the trailing edge of the clutch shoes)creates a more positive engagment of the clutch reducing clutch slip which reduces heat build up, and cleaning and scuffing up the inside of the clutch bell for a more positive clutch engagment. You might even try a new glow plug, believe it or not, but a worn out glow plug can lead to enough power loss that it over works the clutch to push the existing gear and tire combo. I run vented clutch bells and flywheels on my vehicles to further reduce heat build up in the clutch. Also, if you run a tall gear ratio or have large tires on your truck it will put more stress on the clutch and heat things up fast as well. Hope this gives you more insight on things.