I'm also gonna throw this out there, look at your firewall from the engine compartment side and be sure you have holes there for the evaporator core lines. I had a customer years ago that wanted me to add A/C to his later model GMT400 that was an AT&T fleet truck; there were no holes there so they would have to be drilled. If that's the case you'll want to use the donor truck to make a very accurate template so you get those positioned correctly.
If memory serves correctly you have 8 fasteners to remove to get the box out of the truck. Most of them are black screws with a 10mm head and large integral washer; these go right into the plastic of the box. A couple of them are nuts like where the accumulator bracket attaches and also I believe the one under the heater hose connections where the ground strap from the engine fastens. The one that will evade you if you don't know about it is a bolt from the inside of the cab, the only one, and you can actually see the end of it sticking out right between the top evaporator core line and the fuel pump relay. A long extension and 10mm socket will reach it.
Red arrows pointing to the two evaporator line holes in the firewall, red circles around the fastener locations, with exception of the blue circle around the one bolt accessed from inside the cab. Hope this helps.
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EDIT - LOL, while I was marking up that pic, you were posting that you already got the box.
Richard