your strongback whether internal or external is the basic foundation of your finished kayak. If you plan on building it, construct it with care. Nick R.
If you are using an external strongback, it doesn't matter at all if it is straight or flat as long as it is stable. It's the position of the molds that's important, and with an external strongback the mold positions can be easily adjusted over a wide range. That's exactly what the impatient builder finds exasperating with an external strongback - it takes some time and patience to get everything aligned.
With an internal strongback, the molds 'slide on' to the 'beam' and there's not much latitude for adjustment, OR need for adjustment as long as the strongback is absolutely straight and untwisted.