Most aftermarket systems use the Sanyo/Sanden(or others) because of the small size. They do the exact same thing as the A6 or R4 compressors, but just take up less room, which makes it a little easier to install. They also seem to use a little less horsepower in the process.
I would assume the only problem with using the A6 compressor would be getting hoses to mate with the aftermarket hardware. You would prolly have to have the hoses made up to work with it, but that can be done at a competent shop.
The brackets to mount the A6 compressor to your engine are still available, if not new, then used, so that shouldn't be an issue. Then you'll need the pulleys and belts.

Thank Joel for your help.  I found a place on the net that sell a converter for the house.  The cost was $24.  I was really happy to find it.  Just got a few days ago and it fit really well.  I just got to located a big block AC bracket now.  Good thing it's still cold out there.  Hopefully when it's all said and done it will be worth all the extra effort.  Thanks again.