I drilled 5mm holes in my forms in exactly the same spot on all of them. Alignment was pretty easily achieved just by clamping a level onto the centreline of each form as I aligned it, and then moving them until the holes all lined up and the level was true. I had a desk lamp shining from one end, and by the time I had finished I could "see the light" down the full length of the boat on both sides. The alignment of centres, and rocker is a given once the holes line up and the forms are level.

A square strongback is definitely something to aspire to, but not absolutely essential. A stable strongback on the other hand, is vital.

: Exactly what I was thinking. I do a lot of woodworking besides boats. Wood
: moves, that's just part of working with it. I also agree that you 'could'
: use it twisted.

: However, I sometimes place a framing square on the strong back to make sure
: that the frames are square, I place spacer blocks under the keel to make
: sure the rocker is correct. I measure off the strong back to check
: dimensions, etc. So if it were me, I would rebuild it and make it right
: because it is my reference point.