a 27.0 crown race doesn't mean the headset is necessarily JIS. JIS uses 30.0mm frame cups. M
Sounds like your new fork uses a 26.4 mm race, so obviously you'll need a headset with a 26.4mm fork crown race. But you'll want to measure (or get a member to confirm the size) of the existing headset frame cups. They might be 30.0 or they might be 30.2. That .2mm is enough to cause you major grief.
When you go to order a headset be sure to specify which size fork crown race you need. You can get 30.2mm cups with either 26.4 or 27.0 for crown races.
Sheldon Brown's headset crib sheet is misleading. It can lead you to beleive that an "ISO Headset" is always 30.2mm cups and a 26.4mm crown race. In reality you can get 30.2 mm cups with either size fork crown race.
I'm not sure if you can get 30.0mm cups with a 26.4mm fork crown race though.
__________________
--Don't Panic.