I have been tinkering with AC for my WB van. I am trying to fit a modern compressor down low on the RHS of the engine but you seem to have a solid plan for yours. I wanted to create two different zones (i think this is what you are after) and i wanted the rear zone to be run from an electric compressor mounted under the van. Anyway, that got too hard and i am back to running a single engine driven compressor with two separate evaporators, so i get two zones but the engine has to run. One is the factory air from a WB Caprice which would fit perfectly with the dash you have and its integral plumbing. The other is an after market evaporator unit that could provide air for the rear if you could install return air ducts to where its mounted (void areas should be ok) and duct the four outlets to where you want them in the rear. See pic below

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