Howdy Michael Rosepal,The hole is on the bottom. Should I use fiberglass resin for the hole repair?
Should I drill the rivets out of the pressboard and remove it? The plastic is in good shape so I will clear coat it. Any suggestions on which type of clear coat? Brush or spray?
As Always Thank you!
On the hole, and yuppers I wasn't orientating it properly in mind and thought it was atop, I would use fiberglass resin and cloth since the hole is a decent size hole and the cloth will keep it from cracking. As for drilling out the rivets, it's a crap-shoot because you have the propensity to do more damage that way, especially if they are those rosette style rivets. I haven't paid much attention to a heater only 3rd gen full size Ford because our chosen cars are factory HVAC ones and the heater only parts car systems were just pitched. I am currently working on the interior side HVAC plenum and I had to drill out countless rosette style rivets and even being careful it's easy to cause more damage. You'll have to make the decision based on your confidence.
For the clear coat, you can use a rattle can variety for convenience as your sealing it and not building up a thick layer.
I forgot to answer your question on the blower, I apologize it was very late/early morning when I read through it and my brain was shutting down. I would bead / coal slag / sand blast that blower wheel then spray paint it. If your coats are reasonably even it will be in balance. Also I would strongly encourage you to go through the motor and if anything grease/oil the bushings as they are going to be very dry by now. This also gives you a chance to check the brushes and commutator as well as dress it off. This is important because the blower is buried under the wing and you do not want to have to service that again, at least in your lifetime
To Actions point about a drain hole, even though yours in heater only. I did notice on the factory HVAC versions of your car the heater core plenum has a small drain hole in it. I would assume this is for two reasons; first being since it's right in the fresh air inlet from the cowl any water splash that makes it through the inlet hole and through the heater core needs to drain out the engine bay side so it doesn't end up in the interior cabin. Secondly in case the heater core develops a reasonable small leak it can also drain out the engine bay side rather than the cabin. It's about 3/16" hole. If yours doesn't have one it may be a good idea to add one in the low point.