Get yourself a 4" deckplate and a 4-½" hole saw and make yourself a cleanout port for the heater box. It's no wonder I didn't have any heat this winter! This is inexpensive and takes about 20 minutes, everyone with a first gen should do this.
Here is what mine looked like.

Bad

Good

 

But, but, but.... I was SAVING that stuff for LATER!

I can see why your heater performance was sub-par.

 

For real...

 

It's better to keep it from getting in in the first place.

 

of course, but most of us bought our trucks used and they're 40-50 years old, so no telling the life they lived. It's still a good idea to add this cleanout if for no other reason than to clean out the junk that's already there.

 

My 78 didn't have any of that when i restored the hvac unit. It did have a rusted out pan that the coil sits in. I made new one out of stainless steel. Guess it depends on where the truck was parked during it's life. I always keep the cowl area clean of leaves/debris that might get in there.

 

P*ss on the trolling parrots.
That....
is a good idea.
Nicely done.