If you just had the heater core replaced, I would take it back to the place that replaced it and have them check it out. I would think it would be under some kind of warranty. They might have forgotten to plug in the blend door motors or something. Why did you have the heater core replaced? was it leaking or was it because you didn't get any heat from the heater before?
after your truck is up to normal running temp, 99C vs 210F, check the two hoses that go to the heater core out in the hood area. Are both hoses hot? (do they feel like 99C coolant is flowing through both hoses?) If one hose is hot and the other hose is much cooler then you either have a blockage or an air pocket and the coolant is not flowing properly through the heater core. If both hoses are hot then you should look at the blend door operation.
most of the time the air will eventually get pushed out by the water pressure at higher rpms. (2500-3000) It may take a while because it has to be with the thermostat open and higher rpms to push the air out and through the system.
You could put in a flushing tee into the heater line and manually fill it with coolant.
see if there is someplace that will rent you an air evac tool. Draining the coolant out and saving it, then using a tool to evacuate the air from the system and then letting the tool put the coolant back in is usually the fastest way to get the air out.