[hatzie]

The original engine in your truck was dressed with a Frigidaire Harrison A6. I'd try to come close to the specs on that beast for dual evaporators.

The Harrison A6 compressor was a nearly indestructible beast. It will push out 27,000 BTUs at 2,000 rpm and as high as 42,000 BTUs at 4,000 rpm with a discharge rate of 240 p.s.i. That’s enough cold air to cool a small house (my window-mounted air conditioners are only rated at 5,000 and 8,000 BTUs).
They don't care whether you turn em clockwise or counterclockwise they'll work in either direction.
Not fuel efficient at all but they moved so much refrigerant around that it would freeze you out of the passenger cabin when the system was properly charged.

The compressor that GM put in the 3/4ton suburban with the 6.0L and dual air using Envirosafe refrigerant may get you close to the performance of the A6 with R12.
Be aware that Envirosafe is a hydrocarbon blend refrigerant. For those that can't help but mention that hydrocarbon refrigerants burn... R134a & R12 burn too. The byproducts of automotive refrigerant combustion are really nasty. Hydrocarbons don't generate Phosgene or Carbonyl Fluoride gases when they burn. I choose to not melt my lungs and brain.