Have you removed a crank bolt to see how much distance there is between the washer flat of the crank and the end of the spindle? Theoretically (ISO standard) there's supposed to be min. 1.5 mm when the crankarm is torqued down. Sheldon Brown's spindle compatibility page says that he mixed JIS and ISO without too much difficulty.
http://www.sheldonbrown.com/bbtaper.html
So what I'm suggesting is that if the distance between spindle end and washer flat doesn't look dangerous to you, like 3 mm rather than 6 mm, and you'd like to stick with a cartridge bearing BB, just buy an inexpensive IRD BB that's shorter than the one you have: measure your current chainline and chainring-frame clearance, figure out how much you'd like to move the crankarm inward, and just get a unit that's twice the difference shorter than the one you have.

If the spindle end/washer flat clearance looks scary, then go to plan B: ISO spindle and loose-bearinged BB.