In general, 26.4 mm and 27.0 mm crown races are identical for a given make & model of headset, with the exception of the diameter of the crown they seat on. If your fork's crown is 26.4 mm, use a 26.4 mm crown race. If the fork's crown is 27.0 mm, use a 27.0 mm crown race. The different diameters of crown race (again, for a given model) don't require different bearings or cups.
BUT... There are multiple standards for the diameter of frames' head tubes, just as there are for fork crowns. 30.2 mm is the common standard, but JIS headsets were designed for 30.0 mm tubes. If you're replacing a headset, measure the outside diameter of your fork crown and your old headset cups. Then be sure you buy a headset whose cups and crown race are the right size for your bike. (I've seen some odd combinations of ISO and JIS, so it's better to take a measurement than to make an assumption.)