The problem with using dimension lumber as a strongback is that there is a better than even chance that it will warp as moisture content varies. I bit the bullet and built a 2" x 6" box beam from 1/2" MDF (RangerBoard, I believe the trade name was). It has been extremely dimensionally stable. It's the same stuff that the forms were cut from. It is a bit heavy, but it's rock solid. No warping.

: Your idea was valid. What you have to look for in that case would be a fairly
: straight grain, either flat or verticle. You want to avoid the pith or
: center of the tree, which is what most of the deminsion lumber ( for
: building ) is. If you DO end up with the boards with the center in them,
: then on each side of the ladder beam, put the curved grain in opposite
: directions. That way, as they dry, they will pull against each other and
: cancel most or all of the movement. Don't know how you were putting them
: together, but instead of nails, you might use lag screws with fairly big
: washers. They will pull it together and hold about as tight as can be
: expected. FWIW

: Rehd