A 10" spacing means 10" between the bits of paper, therefore mounting the MDF forms with the paper side facing either end (i.e. facing the stern behind the max width point, and facing the bow in front of it). With 1/2" MDF, this will give you a space between the two middle forms of 9" and 9 1/2" between the rest.

In terms of the strongback length, if the Bootlegger is anything like the Guillemot, the first and last forms are not mounted on the ends of the strongback. The strongback pokes beyond each a little bit, to provide fixing points for the end forms.

This video (from Nick) shows him constructing a microBootlegger, and this shows the forms.
https://youtu.be/KuvZcja6kDM?t=149

I'm only speculating of course! But that'd be my guess.