one thing to look out for is the ends of the 2x4, boxbeam etc to be on the same plane. If one end is twisting one direction and the other end the other way you ends will obviously be off. For the end forms it easiest to get a straight boat if you have a flat straight top to mount them on. the beam can bow from side to side because you can adjust the forms to compensate but needs to be free of twist. on my first boat I cut down a 2x6 because I couldn't find a straight 2x4. this one i built a boxbeam. It works better but it can be hard to make it perfectly straight over its length. It will be straighter than most 2x's.
I used a four string alingnment on this boat and it helped out tremedously. i drilled four holes through each form and ran string through all of them. the boat is dead straight. This was more important than a perfectly straight beam.

Tom

: An ordinary 2x4 is working out fine for me. If you store it where you are
: going to build the boat for a few weeks, and it doesn't warp or twist, it
: should be ok.