I ordered the complete Great Auk kit which shipped with the building frame (you can also order the kit without, for example if you are building a second boat).
The strongback is the part of the building frame that runs the length of the boat and holds the forms in place. It needs to be as straight as possible so the kit includes some pre-cut 1/2″ plywood to be assembled. Plywood is easier to keep straight and true plus easier to ship in sections. Finding and shipping a 16′ 2×4 that is straight with no crown or warp is likely impossible.