Jeff,
What your trying to do is interesting. The issue is how automatic you will need the rig to be.

Speaking from skiff experience and zero knowledge of the ARC range.
Skiffs are much more tender than any cat. I suspect this will also be true of your sports boat. One sport boat (Boatspeed 23?) down here is a filled (the hull is extended to the end of the old rudder gantry) in 80s 18teen with short wings 3 on trap and a number 3 18teen rig).

Cats have a higher inertia and you may find you stagger rather than accelerate. (Im not putting ARC products down here).
I know there are some 18teens in california and perhaps they have "old" rigs available. Alternatively you can pick up rigs from the 18teen teams in Au cheaply. They are designed for mast heard kite forces without backstay. I believe the sticks are now being made by CST (composite spars and tube in Sydney amongst others). These rigs also flex under kite loads and thus automatic downwind as well.

What I would be inclind to do is get a rig designed for the boat. Wont be much more expensive than purchasing a new ARC rig. You will get better balance for the boat in the end. Measure everything the rig designer will need everything you can provide.
Which ever way you go expect recutting, thus get a sail maker who has interest in your specific project. If possible find one close who will come out and both crew/skipper and visit the boat.

Good luck

Stewart