Sharing for those who might find themselves in the same boat:
Truck in subject has 305 motor with A/C. It is a run-around vehicle for me and not used much. It has about 175,000 miles on it.
About two months ago had smell of coolant in cab, steaming windows with the heat on, and a couple of coolant drips on passenger side. Easy fix - heater core. Picked up new core and removed old one. Looked perfect. Pressure tested without leaks. Reinstalled OLD core as new one was wrong anyway (must have been for truck with no A/C). Checked all radiator hoses including heater hoses. Losing a little coolant now on a regular basis.
Finally ran truck under load, with heat and a friend revving the engine. Found a crack in the radiator near the lower hose that was shooting coolant RIGHT ON TO THE FIREWALL above the heater core hoses. Coolant was following the hose and piping right in to the cab.
The old radiator was cracked on the barbed fitting for the lower hose and was a real bitch to find.
It might be a 1 in 1,000,000 thing here, but if anyone has a similar problem, don't ASS-U-ME as I did.