I am building a Great Auk with a steel stud strongback and have had no trouble with movement or stiffness. Just a couple of things to keep in mind....

: I am building a Guillimot and have had three tries at constructing the
: strongback out of 2X4s as suggested. I was able to keep them straight by
: clamping the two lengths together for a couple of weeks before
: fabrication. But I could never get them not to twist excessivly. So
: finally I tried using metal studs available at your local building supply.
: These items are available in 8 and 10 foot lengths and in standard(thin)
: and heavy duty (thicker) gages. They have a cross section which is U
: shaped with a little lip on each edge. I found that if you cut off the
: little lip on one of the edges you could join two of them together to form
: a 2X4 shaped hollow beam.

The heavier (20 gauge, I believe) top or bottom plates have no lip to cut off and make a stiffer strongback

You can join them together with sheet metal
: screws and use a short length of wood 2X4 as an internal form to keep the
: dimensions right. I joined two of these hollow sections together with a
: wood 2X4 section about 18 inches long.

If you cut one of the ten foot sections in half and join it, face to face, with a full length piece, overlap the difference with a whole piece and then finish off with remaining half piece then there will be no need for wooden splices. The butt joints on either side of the strongback will be five feet apart and will not show any flex. (The description is a little weak but listen to what I mean, not what I say)

The end forms can also be attched
: to wood 2X4s and inserted into the hollow ends.

This would work fine. I used a metal cutting blade in my skillsaw, tinsnips, duct tape and a handfull of bandaids to get the tapered ends on my strongback.

The big advantage is these
: hollow steel beams are straight,and if they are not straight you can bend
: them to straightness and they will stay there. The disadvantage is that
: attaching the frame sections is a bit more difficult. You have to use
: predrilled holes and sheet metal or dry wall screws to attach them. If you
: are having trouble getting a wooden strongback to be straight and true you
: might like to try this idea.

One final thought. Since the sheetmetal screws that hold the strongback together are on the top and bottom, if you premark the steel for your forms you can avoid having to relocate screws that would be in the way of your wooden forms.

I have less than $15.00 in my strongback and plan on using it again