If you pull you heater box I am sure you know the dash needs to move out of the way with AC. Take seats out and console if you have. There are only like 8 fasteners not really that bad to pull dash back. Got to WCCC West Coast Cougar Classics they have video on heater box rebuild. You will need a foam kit and I add a piece of 1/4 mesh wire between the cowl and the heart box to keep the critters out. Nobody can see and it works. I also put the mesh on the back side of the plastic cowl vent grill use small zip ties to hold. Then I cut a small piece of stainless steel pot scrubber pad in the cowl vent drains so they cannot put nest inside the cowl. Bad pic of the mesh on the heater box.
Also when you take the doors apart inside the heater box support around the shafts so you do not break the box. Also the drain hose will break the connection easily. I have a long 3/16" drill bit I run in the groves in the doors for the wire forms makes them go back in easier. Might want to replace the blower motor also. I did not and just after back in the motor started to squeal.
If your heater box does not look good wash with lacquer thinner and spray with satin clear will look like new. If it has been painted remove the paint first.
You might want to take the condenser and evaporator to AC shop and get it flushed. If you have not changed out the O-rings to new good time to do that. Also new schrade valves at the service ports. I use Duracool much better than R134 or R-12 and you can order online.