Just tell the Classic Air folks that you want a system that runs on R134 refridgerent, and the Gen III Hemi compressor will work just fine. If you swap to a spool mount k frame from a 73-up A-body, you can make the compressor fit in the factory location on a car motor. See my thread to see how I made it work.
http://www.forabodiesonly.com/mopar/showthread.php?t=173963 If you use a truck motor, the AC compressor is up out of the way to start with. The car Hemis make a cleaner looking install, but you have to fight the AC compressor. Good luck.