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I'd set them up 90 degrees apart, selecting the location after scouting the best shooting direction. The Sto-Fens will eat up some power, but given the high-ISO capabilities of the Mark III the 430s may be able to handle it. Aiming the Speedlites into the shooting area will give a mix of direct light and bounced light. (Usually I put my Speedlites in 90-degree tilt when using the Sto-Fens, but if you keep them in direct orientation you'll have both direct (shoot-through) and bounced light.)
Are your radio triggers ETTL capable, or the flashes at least adjustable from the camera? If not, you'll have to ride the apertures to get proper exposures from varying flash-to-subject distances, or pick a "subject zone" within which you will have exposures within a range that can be easily adjusted during post processing.
Good luck, and have fun.