The BB on my Cannondale ST-600 is a sealed Sakae SX unit (English threading) and the spindle is labeled 3S (121.5mm length). See http://www.sheldonbrown.com/bbsize.html for more information on Japanese BB spindles.

About the chain falling between chainrings, is it possible that you're using a too narrow 9/10-speed chain instead of a wider chain for the stock 6 speed drivetrain on the Cannondale?