Derek,
Like Mark, I am also confused as to your problem. If you plan to use an internal beam, then you need to find a good 2x4 which is straight as possible and the length you need. That can be difficult. As for the length of 14' plus, for the internal, you don't need it length of complete boat as your end forms will extend perpendicular to end of beam.

If you use external, then any sheet of ply (usually 1/2) can be cut to desired widths and built into a box beam. The construction and edge cuts will ensure that it is pretty straight. Any slight deviation can be corrected as you lay up forms on risers.
Good luck
Rod