I am about to try another idea. I bought 2 10' steel barn door rails at Lowe's. They are heavy, but very rigid and straight. I'm joining them by bolting three steel mending plates across the joint. I plan on mounting the forms above the strongback, so I won't need to do any cutting of the beam.

Even with a good beam, I still plan to use the string-through-the-forms method of keeping the forms aligned. I'll also try using wedges to secure the form supports to the beam.