I just spent a little time investigating this conversion. In a nut shell, it looks tough.

The 1991 and later HVAC controls have no mechanical connections to the car, just electric and vacuum controls. The wiring harness connections are totally different, and the vacuum tube connection is a linear rather than a radial manifold. The 1991 and later heater used different vacuum servos and different electrical connections. The heater ballast resistor is different. The HVAC wiring harness appears to be somewhat integral to the chassis wiring harness.

I think this converision could be done, but you would want to have a complete donor car available and the factory service manuals and wiring diagrams for the HVAC controls for both cars and the ability to understand these factory service manuals in every subtle detail.