I just starting to build a Great Auk double myself. I used almost exactly the same method you did. I put an alignment sting on either side of the forms and clamped thin pieces of wood along the datum line than runs across the forms at the top of the strong back. I measured off one sting back to the strong back for the centerline then I just lifted the forms to where the line extensions just touched the bottom of the stings. I clamped them there and predrilled the screw holes so nothing moved when screwing the forms to the brackets. No hassles. The only real change I made was by making the center hole in the forms much larger so that the strong back wouldn't have to be perfectly true.

I placed the strips on the shear line and suddenly it looks like I'm building a boat. BTW- this took a lot longer than I expected it to.

--KT--