is 2x6 not a little overkill for hive lumber? Perhaps you could barter some of these for some 1x12 stock and save yourself the trouble of dealing with the crack between the boards, warping, etc. If you want to use the 2x6 boards, you could always router them or use a dado blade to make these tongue and groove, and then run a little glue in the groove before putting them together. That way they will stay better in line.
justgojumpit