You will have a tough time cutting the center piece with the circ saw. Think of the max depth of the circ saw (probably @ 3" per side) and you're going to have a large 4-5" section of the center part of the beam that won't get cut. The sawzall could finish the job there.
I would strongly recommend renting a torch for a job like this.
Is this cut needing to be pretty, or just cut? Sawzall would do it all with enough blades and time. It may or may not be pretty either way.
