I know nothing about MTB bottom brackets or 73 mm BB shells but - does the taper of the spindle match the taper of the crank? You know that (at least for the road, again I know nothing of MTB stuff) there are two taper standards, JIS and ISO. JIS is the Japanese standard and is used for nearly all Japanese road cranksets and BBs. ISO is the international standard used by Campagnolo and is the European racing standard. You can put a crank made for one taper on teh other, but the crank will sit in an entirely different place and chainline. (See Sheldon Brwon for more discussion and numbers.)
There is one Japanese crank/bottom bracket that is made to the ISO standard (and probably others I don't know about). Sugino 75s, made so they are interchangeable with Campgnolo and other European makes so the track racer at the velodrome has parts that interchange with other competitors and he can receive parts that work from the infield mechanic.
I'd guess you could use most any 70 mm sealed bearing bottom bracket that does not use a lockring of correct standard for your crank. That would put your driveside crank 1.5 mm too far out, not perfect but entirely within reason. This works as long as you have 3 mm clearance to spare with your non-drive-side crank to the left chainstay.
Ben