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@malandro
caveat: i'm not an expert and thus could be talking out of my arse
i think what you're saying is that you're trying to change the bottom bracket on what i presume is an old bike from that 'cup and cone' style BB (the one pictured measuring 120mm?) to a more modern sealed cup BB?
you've measure the BB shell - 68mm - and so any BB you buy needs to be 68mm by 'X' where 'X' is the length of the spindle. this is usually determined by the cranks you buy. if you by a modern set of cranks they will usually tell you the minimum spindle length you need.
eg: the following crankset (chosen at random)
http://www.tokyofixedgear.com/products/50-Crank-Sets/740-Sugino---RD2-Messenger-Crank-Set---White/
tells you you need a 103mm square taper BB. So any BB you buy (if you were buying these cranks) needs to be 68mm by 103mm.
if you're putting the old cranks back on and you want a new BB i'm guessing you'd need 120mm x 68mm but this might be because the bike originally had more than one front cog?