Bear with me on the response to this thread. This subject has been discussed many times over the years.

You really have to ask yourself how crazy you want the heater box resto to get. I agree with what everyone else is telling you and that is to do it yourself. If you choose to do a full tear down, you can get rivet squeezers and different size heads for not a lot of $. Keep in mind you don't need an aircraft quality squeezer to get the job done since the rivets are semi-tubular that you will be installing. Having a rivet squeezer is also helpful especially when installing the hard to reach ash tray rivets on an e-body. if you don't want to buy a rivet squeezer you can fab one out of a large c-clamp and just buy the dies.

If you decide not to tackle the project yourself, Instrument Specialties is more than qualified to do the job. I can't imagine a fast turnaround time because of how big Mike's operation has gotten. Keep in mind there is a price for perfection when dealing with shops worth there weight in quality work. There are many...

When I redid my heater box many years ago on my 70 Cuda, i realized the only way to do it right in my opinion was to to fully disassemble the box. Rivet removal and all. Once I had everything disassembled I went through every part to see what needed to be sent out. At the time (2009) it was a no brainer to send the fried heater motor out to the Late but Great Jules who was located outside of Montreal. I checked the blower motor resistor and it was good and just needed a very light cleaning to look new again. With a heater box that is not too jacked up a diluted concentration of simple green works wonders cleaning the fiberglass sections and does not mess up the stampings on it.
DMT was a fairly new company in 2009 and had the best quality gasket set for mopar heater boxes. I believe nothing has changed with them and they have continued to up there game since that time.

Now for the fun part and this is where the insanity begins---->>>>the lovely fiberglass repair!!!! (if needed) IMO the only way to fix damaged sections of these boxes is to find good condition sections of other heater boxes to rivet together with the good sections of yours. (These pieces do pop up on eBay from time to time.) If you can't find the mint sections then you will have to master the art of fiberglass repair which does take some practice. The problem with FBG repair is "the blend" to make the repair look invisible and the box to look factory again. This means you will have to use a paint mixture to dust the box to hide the repair work. "Resto Rick" has a good formula for this on his website as well as recommendations how to clear coat as well. However this ultimately changes the look to a somewhat non factory appearance. It does not ruin the box but it is noticeable. The good thing is that the box sits under your dash and from a few feet away most won't recognize the repair and repaint if you go that route.

Hopefully you are in 5% crew and your box just needs a light clean up. Most do not have a nice HB and have the infamous broken vent opening closer to the passenger side that people used to rest there feet on. LOL. I wish you luck on whatever route you decide.

Some side notes:
-Im pretty sure any FBG repair would have to be done with short hair FBG. its been a while so i don't remember exactly which one i originally used. You can mix oil base paint to tint the FBG.
-my brackets on the heater box and driver side vent appeared to be in natural stamped metal, if your not sure you can show the plater and they will know. (if natural metal use RPM to protect, clearing it will leave a yellow hue after a while)
-rivets are super cheap and you can get the exact sizes to replace the rusty ones.
-If you need to put the paint stamps back on the box, you can have them made fairly cheap or borrow from another moparts member that has been through this. If you have stamps made keep in mind you have to clean them before using due to the mold release agent on them.
-Remember there is no school to go to on this stuff so the only way is to get your hands on it and try your best. Ask questions, there are a lot of knowledgeable people out there. And as always if someone has some info to expand my response or correct any bad or outdated info please chime in.