I installed the kit by Classic Auto Air that NPD sells. It works great, looks original (except that it's an aluminum compressor and it sits where the alternator is supposed to go). The alternator gets moved above the compressor and is run from a small v belt running up from the compressor pulley. Its a rinky dink set up, and I plan to convert it to a two belt system. The main unit hangs below the dash and is self contained. It cost less than a grand and I was able to install it myself (during the restoration). It doesn't rob power and doesn't noticably effect my MPG. The only down side is that the instructions SUCK! They had a nice step by step instruction sheet, but it was for the V8, not the six. Likewise they included an install article from Mustang Monthly, but it too was for a V8. Tightening the belts requires loosening five bolts, three of which are hard to get to. Because of only having one belt I have had to tighten it often and have replaced the belt three times in 22000 miles. Don't fear the bad instructions though, you can call the tech line and they are very helpful.
Harry