I have heard you can build your own system on the cheap, I have not tried this yet, as I am still gathering the parts myself, but here is what I have found.
Go to the local junkyard, and find a conversion van. You know the vans that are all decked out with everything but the kitchen sink. 95% of them have an aftermarket heat and air unit in the back by the rear doors hidden under some fancy interior cover. The unit I got is not much bigger than the original heater box from my 41 international pick-up. Get the hoses and duct work while your in the van they are like ten miles long and you can use them for other projects too.
Get a/c condenser from a 1st gen mustang. they are fairly small (about the size of a Model T grille shell opening).
Get the compressor and drier that go with your donor drivetrain, or upgrade as you like.
Take those extra long hoses you got from the van and the fittings from all your other equipment, cut them to fit, and take them down to the local hydraulic hose manufacturer to have them compressed.
Hook it all up one last time, start the motor, turn the A/C on, pull a vacuum (approx. -25) for about thirty minutes, charge the system with Freon, and live the life of a/c in your ride.
Again I have not tried this yet, as I am still gathering parts for the build, but I do have the parts to put A/C in my rat rod. Hope it helps someone
Build coming soon to a thread near you.