I would never use a simple 2x4, they all move or because they are wet when you get them or because they are green, there are knots and, who knows how many other reasons there could be, you are going to find them in a 2x4.

I recently opened a wall in my house, brand new, and found a 2x4 twisted almost 90 degrees, and im not exagerating, to the point it cracked open where it was nailed to the header, so go figure.

Id rip it by the length as best as you could, reverse one half and reglue it together, that to try to keep it from moving, at that point wont be exactly square so you still need to snap a reference line in the middle

The absolutely best its using a ladder as strongback (external), aluminum or one old wooden ones, you could still try to firmly clamp your 2x4 to a ladder and go on.
Building a plywood box its sometimes considered extra work if you will build only one boat and dont have space where to put it for the years after (if you build a box build a triangular section one), but if i remeber well someone built a box with cardboard sides and had a pic of it on two sawhorses and him standing on the middle of it (will look for the link), maybe check that out too

Whichever way youll go if you use that 2x4 keep good reference points and check them often while you strip (a laser level can be amazing for that)

: I am just starting my build of a guillemot and unfortunately the 2x4 i bought
: for the strongback appears to have twisted along its length! Any
: suggestions as to what would be a good course of action to take would be
: appreciated.