Removal of heater box

A factory service manual is your best reference for this and many other proceedures.
You may find the manual as free download here somewhere but if not it will be worth every penny spent if you plan to do these type repairs yourself.

The advantage of a forums members and their experience is best limited to tips.
here's an example... Someone may have mentioned a J bolt that goes through the right vent box from the bottom up. It hooks to the cowl and draws the vent box up.
They and the service manual wont mention that the bolt has a predetermined thread length so it cant be over tightened or even tightened a tad more than before removal.
The service manual wont remind you this is a 30+ year old car and that vent box seal will probably need replaced. It's a 5 dollar piece at Detroit Muscle Technologies.
To clean the surface with alcohol before attaching the seal helps but the adhesive supplied on that seal really is pitiful. It will peel off the seal about as easy as its paper backing peels off Use 3M weatherstrip adhesive instead and let it cure a while so the seal stays in place.