Try and old trick for an old truck. Go buy a box of Arm and Hammer Washing Powder. Not baking soda, Washing powder. I dont know it they still have instructions on the box. Here is what I remember, drain the system of coolant, as best you can, remove the thermostat, reinstall the thermostat housing (no thermostat) install 2 cups of washing powder and 2 gallons of water leave off the radiator cap and crank the engine. Run it till it over heats. Or for about 15 min. Drain out the gunk and do it again and again untill you only have soapy water comming out the drain. Then one more time with just water no soap. Drain the system, install a new thermostat and antifreeze.
If the problem still persists you'll have to remove and dismantle the engine and vat that sucker in acid.
Good luck.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"