I have done a little kayak sailing. 1" inch of rocker fore and aft is typical for a recreational kayak. It won't effect waterline much since you're lifting the ends which don't have nearly as much bouyancy as midships.

If I recall the Blandford designs have geodetic ties to stiffen the frame. However it is difficult to hike out with a kayak and a SOF is not stiff or strong enough to handle serious stresses, so the amount of sail you will be able to carry safely won't be much unless you use floats. Depending how exciting you like life to be, 25 sq ft is probably enough to handle from the cockpit; 15 sf can flip you in a gust with some kayaks. That's barely enough to tack even with a leeboard and rudder, but upwind performance is a dog. However that will move you along smartly on a reach or a run. Remember you need to react the sailing forces to you, try not to run the forces through the hull, and have a stand-alone mast - no standing rigging.

Sailing canoes can carry 100+ sq ft of sail but they're a whole different craft, for that kind of sail area outrigger floats are a must on a kayak. If you have floats sailing forces will have to be reacted by them as well as by you. When sailing try to keep them out of the water, run with them in the water and they will slow you down, unless you go for serious sail area. They're just to give you time to balance and stop you flipping in a gust or following a bad decision.

If you want a SOF sailer and haven't built the kayak yet why not consider a SOF to a proper sailing design? Simpler and probably lighter than a kayak plus floats plus crossbeams and can carry decent sail.

Example here (scroll down for image of frame):
http://www.boatdesign.net/forums/projects-proposals/challenge-100-boat-12176-7.html#post198820