Thanks for the tip on the hoses, Funola. I hadn't thought to use hot water, but I did pick up that same set of hose tools.
This weekend I finally got back to work on the AC. I focused primarily on rebuilding the evaporator/heater housing, I'm waiting for some help to re-install it since it's bulky and has some fragile parts that I'm worried about breaking during the reinstallation. Essentially I'm roughly following
PeachPartsWiki: Replacing the A/C Evaporator. The housing I am using is a Seimens unit for the evaporator half, with my original Behr heater core housing. I believe the heater core housing is made by Behr regardless of which evap housing you have, as it interchanged easily.
I cleaned everything with some parts wipes, followed by clean water to remove any residue.
I used 3/4" wide by 1/4" thick foam weatherstrip to seal around one side of the evaporator to prevent air from bypassing it. My original Behr evaporator had one strip of weatherstrip on it, while the Seimens housing had residue that I believe was from two strips of weatherstrip. I got the Seimens housing used, so I don't have pictures of it being dismantled to be sure.
I used
Armacell 2 in. x 30 ft. R-1 Foam Insulation Tape-TAP18230 - The Home Depot for the flaps, it seems to work well, although I have no idea how long it will last. It's fairly obvious where the old foam was via the residue left behind from the glue, or on the fresh air flap there was an area that was coated with something black.
I also used some butyl rope caulking to seal around the ports to the evaporator. I wasn't sure what the original sealant was, and didn't want to risk using something with solvents that would harm the housing or evaporator.
I replaced the main evaporator drain with some 1" heat-shrink and 1/2" PEX tube cut to about 3" long. The idea was to hold the heat shrink in place over the drain, put the PEX inside it, then shrink it into place forming a watertight and hopefully long-lasting seal. I also added a zip tie around the heat shrink just in case.
Here's a few pictures of the rebuild process, hopefully they will help others. I'll try to get pictures of the old Behr housing/evap once I have room on my workbench. Let me know ASAP if you need any specific pictures of the Seimens housing, I'm hoping to reinstall it tomorrow night.