Andreas--

I think your math is correct. I hope you're a thin guy for squeezing around the ends. The biggest disadvantage will be not being able to step back from the kayak to sight down the length of the boat when aligning forms and attaching sheer strips. If you plan it right, and especially if you build with the Guillemot internal strongback, you be be able to move from one side of the kayak to the other by ducking underneath it. I do that all the time. If it's a good workspace that you can get your materials and tools into and get the finished boat out of and you have good temperature control, I'd say it's time to start building.

Mike Scarborough
Friendship Kayak