Oh, if you really want to do a serious OBA then grap an air brake compressor from any old international. The duty is 100% and it's nearly 9cfm. There are no shortage of 4600 international dumps, or school busses around. Grab one then just figure out the mounting bracket.
No I was not trying to belittle the AC comp. idea, but pointing out that there are easier ways, and less arduous things to think of!