After being sure all the valves are open, try reverse-flushing the heater core. Adapt a garden hose to the heater system, and run water backwards to its usual flow. The return line for the heater core generally gets attached at or near the water pump, so make that hose the "input" line during flushing. Let the other line run free- - - -don't flush the crud back into the cooling system. Also be sure you install a 180 degree thermostat- - - -a 160 won't get the coolant hot enough to provide much heat to the cab.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!