That is an excellent suggestion, m. I socred a NIB 105 headset for cheaps [$10!], and had the lbs mill the crown race down to 26.4 from 27.0, to fit the headset. I think it cost me something like $10. That was over a year ago, and there have been no problems whatsoever.

I think shimano makes some threaded headsets with a JIS crown race. At least, that's what the spec/instruction sheet seemed to imply which came with shimano 105 headset.