Welcome to Steel Soldiers. This comes up a good bit and several threads have been written about it in the past that cover all the details.
Basically, any stock square body truck a/c system will work. You will have to move some things around on the firewall and figure out a way to mount the compressor though. It will also mean you will not be able to mount a 6.5 GM turbo set up because the evaporator and the turbo need to occupy the same location under the hood.
NAP has a kit with the proper mounts for a compressor so you can keep your 12/24 dual alternator system. However, it is a/c at the vents only. The stock floor and defrost heater is still there and there is no way to stop that airflow coming in during the summer when you just want a/c at the vents.
Vintage Air has a system that replaces the stock heater, adds a/c and lets you put a mix of hot or cold anywhere. It cost more and you will need to buy the NAP compressor mounts separately.
No matter how cold the air at the vent. When it is 100° outside in full sun. You will still be driving a green uninsulated green house. I can have 40° at the vent with my NAP system while getting 130° readings from the floor and 140° readings from the fiberglass top. My face and hands might be cool, but I am hot still.
It comes down to how far do you want to go and how much do you want to spend.
