Car is overheating and heater blows cool air.

POSS STUCK CLOSED THERMOSTAT OR PLUGGED RADIATOR

Did you bleed the air from the cooling system? There are bleeder valves that need to be opened to expel air from the cooling system after you service it. If you didn't you probably got a air lock in the top of the engine radiator and overflow tank will appear full, but you will have anything flowng through the cooling system. Other possible problems are thermostat, radiator or heater core blockage.