John,
Thats just what I'm doing now. I drew up the plans for the Guillemot in Nick's book in a Cad program then scaled up ~6% to get a size similar to the L version and spaced the forms accordingly to get the 18' length.
I gotta agree having never been in the boat it looked like it may be a little tippy for my skills. But after spending this summer paddling my Cape Charles I have gotten a lot more comfortable in a kayak. I probablly don't need the extra stability or the extra volume but it will be a stable boat that I think any one would feel right at home in. I think it might be harder to keep fair lines by laminating material on to the standard size forms but it surely can be done. Good luck with your decision.
Don B.