Unfortunately a lot of the dimensional lumber we get today is not straight grained and may not have been dried enough to stabilize. The answer we have is epoxy or some other potent glue. Use it to laminate thinner pieces into whatever size you want. Because of the grain mismatch between the pieces the inclination of one part to warp is negated by it's neighbor.
This technique shows up all the time in making wooden masts for small boats. Common lumber cut into blanks and glued together can produce a substantial mast. The blanks can even contain modest sized knots. As long as each knot is supported by a clear section of lumber on an adjacent blank the mast will be plenty strong and will stay straight even tho' it's long and thin.

: I am just starting my build of a guillemot and unfortuantely 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.