I think you want the beam to end at the 1 foot and 16 foot forms. If you were going to fair the forms they would sit outside the 1 & 16 station lines and the beam would be 15 feet long. If you don't fair the forms they will sit inside the station lines. You would want to shorten the beam by the thickness of the two forms. If you were using 1/2 plywood that would make the beam 14ft 11in.

To attach the end forms use a couple of cleats about 3/4in. square and 8in. long on the top and bottom of the beam, running the length of the beam. You may have to experiment a little so they don't interfere with forms 1 and 16 or with the hull planking when you get around to that.

You could fill the center of the beam with plywood. I made my beam (solid) out of 4 thicknesses of 1/2in ply cut 4in. wide and screwed together. The disadvantage of that is that it is probably less stiff when flexed left to right than a box beam would be. Filling the center of the box beam will add weight faster than it will add stiffness.