there's so many possible routes here, but i will say that a tange levin cds is a very nice headset. And under $30. And, it looks nice, available chromed or black. Paying a shop to mill the crown to accept a 26.4mm race seems excessive to me, unless you feel a need to install a King or something.
As for the interchangeability, the cups on a JIS threadset are/were .2mm more narrow than on an ISO headset. I've seen many newer headsets with the 27.0mm crown race and the 30.2mm cups, probably b/c all the new, cheap forks out of China are coming with JIS standard fork crowns, with either ISO or JIS-compat headtubes.
-rob (tange levin; can't go wrong)