I doubt it is your water pump.... when it is dumping coolant into the over flow (which is normal as it heats up, actually surprised it has an overflow...) where is the temp gauge? Usually on these trucks the major issues are a thermostat that is not working, or that the rad is clogged. First fit a proper thermostat, if that does not fix the problem, then fluch the rad... both directions to dislodge any scaling. A good rad flush chemical would not hurt either. As for testing the water pump, you can loosen the out flow pipe of the water pump (during warm up, never when it is hot.) , if the pump is working, then the coolant should pour out around the pipe. But as I say, usually the water pump does not fail, the only thing to go wrong with it is the bearings, and theywould let you know, either a loud squeal at the pump or the fan would not be turning due to them being seized.
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Andrew Barr.
1972 Series III 109.
"the Tin Turtle"
"However, that was his ploy and I caved....totally caved...   LOL!" Jellijo 
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