Bill
If I understand your message above your specific aircraft has never had an autopilot? That makes things a little more complicated. Some of the older ARC systems in no longer supported, like my straight 400 is obsolete. I have a great avionics shop that has expertise with these ARC autopilots. I asked them several years ago to make some adjustments on it and after they looked at it he called and asked me if it was still working. I said that it was working fine but felt it could use some little tune up. He said that he was not going to touch it because he could not guarantee that it would still work. I choose to leave it alone and it still works today.
The next item I looked into recently was replacing or overhauling the DG. This ARC autopilot system is coupled to the DG. The DG tells the autopilot which direction the aircraft is heading. Again, as I did research I learned that this DG was no longer supported. It is made by AIM and is a 200-3. There is really no off the shelf replacement for this type of DG because of the connection with the autopilot and the autopilot is obsolete. To use a different DG like a Sigma-Tek would require some mods on the autopilot. My avionics shop again recommended that I overhaul the DG that I have and leave the system alone so that I don't end up with more problems than I started with. Some day when I decide to go the STEC route then change everything but spending the money on the existing system is not really encouraged because it's a dead end.
If your aircraft has never had a autopilot system in it then it would not make sense to put the ARC system in. It would be buying a unsupported component that is no longer is repairable. Just my thoughts after exploring in the last several years.
But don't just take my word on it, call the expert on these units. Bob Ferguson at Autopilots Central in Tulsa is the best guy in the business to talk to about autopilots. He has dealt with ARC for many, many years. He can help you with your options and give some thoughts on the systems that are out there. He can be contacted from their website at:
http://autopilotcentral.com/?page_id=20
Good luck, also search autopilots on this message board to learn what others have used on Skymasters.