That would probably work. but as long as the top is straight and you can get the stems to line up i'd make the holes in the forms bigger. I have always used a router and clamped a lauan template, the size needed to adjust for the bow in the beam, to the forms cut the holes and then lined them up with out worrying. I have never been able to make a box beam perfectly straight, at least not straight enough to get a 2x4 box beam to fit a 2x4 hole and have all the forms in alignment. I would worry about weakening the beam by sawing into it.

and chances are I could be wrong

Tom

: Calculate the amount of the bow and saw some kerfs to match. Make the kerf
: from the side you wish to close up and only saw thru to the opposite side.
: This will allow the beam to close up the kerf. Make the necessary cuts
: where you wish to bend the beam. Have it supported so it will not break
: and close the gap of the kerf to test for straitness. Glue joint and close
: to finish.

: I would expect the kerf to be made at the start of the bend in each location.