So looks like I have to replace my heater core

Not sure why it became accepted to remove the dash on AC cars, at least after about '72.

The factory service manual outlines the procedure to remove the heater core on AC-equipped cars and nowhere does it say take the dash and the HVAC box out. You remove the center vent, the part of the case directly behind it and the heater core is in there. Did it on my '75.

I suggest anyone doing jobs like this invest in an FSM.