The foam seals between the hard plastic HVAC-box shells on my '88 K1500 had turned to jelly. Replace ALL the foam. The difficult one--and the worst troublemaker on my truck--is at the top of the fan area, where the plastic box seals to the underside of the steel cowl.
I used self-adhesive weatherstripping foam in rolls purchased from the local home-improvement store. There's several brands. I think the stuff I bought was "Dennis" brand, probably in 1/4" or 1/2" thickness.
Clean all the debris off the A/C evaporator.
At minimum, blow compressed air "backwards" through the heater core and A/C condenser. Consider replacing the HVAC blower motor.
Getting the dash out properly means removing the steering column. (Don't rip the thin 'n' weak auto-trans gear indicator cable, if used.) Good time to replace/upgrade the lower steering shaft/rag joint. If you're having any issues with the column--ignition switch, turn signal/multifunction stalk, turn signal switch, etc. now is the time to deal with it.
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I have heard that the HVAC actuators get brittle with age. Then the internal gears and such split and become useless. Might be a good idea to replace all of them while the dash is already apart.
Post a photo of all the pens, coins, and other junk that fell down the defroster ducts over the last few decades.