Well, I was able to finish this job yesterday. For me, the most difficult part was figuring out how to remove the console. The four nuts on the backside of the faceplate are an absolute nightmare. Well, the bottom ones are a breeze, but the upper two are nearly impossible to get to.
Once the console was out, the heater box came right out. As I expected, it was piled full of leaves. I went ahead and cleaned and re-sealed the box, lubricated the blower motor, and installed new heater hoses. Surprisingly enough, the old heater core looked nearly new, and it was brass instead of those cheap aluminum ones that are available now.
I was able to get the cardboard plenum out and back in successfully. It had "222" written on the face; not sure what that was about, but it probably had some significance on the assembly line. However, the plenum did tear slightly where the defroster cable bolts up. I guess it was just rotten there. For the time being, I left the defroster cable loose, and I plan on seeing if I can possibly repair the cardboard in the future.
Thanks for all the help and advice!
Here's a picture of the heater box, AFTER I had already tried sticking a vacuum nozzle in the fresh air door and vacuuming out what I could with the box still in the car. It was crammed full; I can't believe it didn't rot the heater core.