I'm not an engineer but I would suspect that the grain orintation would not have that much effect as the joining of the strips side by side is the weakest joint anyway regardless of the grain. You can make a simple piece to test yourself without fiberglass and the results will be dramatic when comparing endgrain to side joints. However in the boats expecially kayaks which have a complete tube formed in the sidegrain orintation the weakness would be almost negated in my guesstimation (is that a word).

By the way, very interesting truck topper. You might think about making those or at least selling some plans. Might paint the top and show the sides to give better UV protection over time, but then all of that pretty wood to hide.