you need to find a carbon fiber pole to place between the hulls at the tangs. From there use two short stays about 70% of the length of the pole each. Then shorten the forestay to meet these two lines in the middle (much like a hobie 20 forestay system. Then run your spin pole between the forward pole (strap spin pole to this pole above) and the forestay triangle. The forward pole stops hull flex and makes the spinnaker more effiecent and saves hulls. I have used this setup on my 18sq with spin for a few years. My 18sq is stretched to 20 feet as a prototype boat. The only problem with the 18 sq without the stretch is that it has limited bouyancy up front and as such is not good with a reacher. If you stretch the hull in front of the forward beam it makes a scary fast boat downwind. The only problem is that the daggarboards are about 18 inches behind where they need to be for awesome upwind performance. (thats why I said its a prototype)

Hope it helps.
Steve