If you have any number of GMT400s in your local Pull-A-Part, it's worth it to nab an entire box. Visit several trucks and assemble a single good box. That's what I did for Roscoe, and I was left with a still very-usable spare box.
I used RTV to seal my box, and it works fine and isn't a total PITA to disassemble so long as you go very thin on the RTV.
Note that there are at least two designs for these boxes. If you accidentally get the wrong version, you can still screw the parts together and get them to work.
I believe you could probably replace the evaporator and/or heater core without removing the box, but I haven't tried.
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