It seems strange, how much I "forgot" about building between last year and this year. I tried building an I-beam twice, using dimension lumber, before giving up on finding boards with no twist or warp. I finally went for this "composite" stuff:
It weighs tons, but it appears to be dimensionally stable. I'm doing an internal strongback again, this time, and after doing it both ways, I think that this technique better insures "straightness".
The "spine". The Strongback Jinx was in-I made the first one 1/2" too small! This is poplar ply. I found that the forms fit a little too tightly, so I actually ran the whole beam through the surface planer. It was a a little bit too much to handle, so I widened the holes in the forms instead.