Hi Dave

the strongback plus endforns is a little shorter than 17' because the strips continue on a little from the end of the end forms.

I too had lots of fun building the strongback! I learns about a minor discrepancy between the depth of the end forms and the adjacent sectional forms (2 & 15) too. It probably has been fixed in commercial plans (I lofted mine from the book) but just in case here is my wee article on it

http://ianjohnsonz.wordpress.com/2009/03/15/end-forms-dont-quite-match-forms-2-15/

good luck for the build

cheers Ian