1969 Dodge Dart GTS - Drain Radiator To Remove Heater Box?

By draining down the rad you will also pull some of the coolant from the heater core.

You could put a piece of hose on the top port to the core look at photos to see if that is the left or right tube or top or bottom. Blow into the tube slowly and you can get a lot more of the coolant out of the heater core. You can use compressed air but keep the pressure low like no more than 16 psi or there abouts