Hi Jeff,
The ARC21 mast is the same section and lenght as the old standard SC20. Carrying a mast head spinnaker will require a couple of 1/8th inch Vectran bachstays. Each backstay will require a little 6:1 mechanical advantage pulley system at its lower end so that the leeward backstay can be eased as necessary. With the chute up the main will likely be trimed down the center of the boat. This is proper trim on a chute carrying catamaran. A close reach is likely to be the point of sailing that requires the 'trim out' position for the mainsail.
What does the Viper weigh? It sounds like you are going to soup it up something like an Australian dinghy. Get yourself some agile and quick crew.
Good luck,
Bill