Thanks for the extensive information guys.
This morning I cut out the 17' forms using your offsets Paul. That works out great as I've already got a 12' strongback. I notched the bottom of the forms for a 2x6 (inverted "tee" type strongback) and fastened them in place. With the 17' plans the forms are 29" apart, thus fitting nearly perfectly into 12'. I'll sweep the ends up a little bit, and add the extra 1" of height that was recommended - but otherwise stick to what's prescribed. Except around the cockpit area, which I may need to raise a little to accomodate my various bits.

Thanks again.