A lot of the first gen trucks have had a flush valve inline with the heatcore hoses. You could run a hose to it and get it done. If you don't have one already, go to any autoparts store, know more than $10 for one.
You could also remove the lower radiator hose from the radiator. Stick a garden hose in the hose, and run the truck with the water on. Have the heater on to. Run for about 30min till it comes out clean water