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

I would drain the system down some, remove the hoses. Put another hose back on one of the core tubes, then with an air gun, lightly blow the fluid out of the core., then cap them some how if you can. There are two or three wires to disconnect and two cable clamps. Theres a hook in the right cowl vent behind the door, and the motor of course, and the defroster hoses. Take a good look at core, it may need replacing. If you mess up the area when removing the clips, you can use JB Weld to bond a broken piece back on or remake the area if someone has been in there and already messed it up
Remove the seat.
Good luck