I'm not trying to rain on your parade but I definitely agree with Dave on this. If you have AC experience you already know adapting a junkyard system can be done but is a MAJOR undertaking. Regardless of what system you use fabricating a decent ducting system and nice looking vents is a huge time eater. Additionally unless you into redoing the system over and over you are not really going to save a lot of money to PROPERLY install the system. At a minimum you will want to replace the compressor, accumulator/receiver dryer, all the hoses and the heater core (it's a lot of fun to finally get the system installed only to have to take it out 6 months latter because the used heater core pucked, or the quality used high pressure AC hose you reused ruptured spraying refrigerant and compressor oil all over the interior).

If you go ahead with a junk yard system the Mustang II/Pinto units are pretty compact. On any of the units plan on spending anywhere from 2-6 hours removing it from the donor car. An alternative if you want to retain a nostalgia look would be to check the 50's/60's cars for an under the dash unit you like they are realty the simplest to install and my experience is they are the more efficient than a lot of the factory units.