I have a (new to me) 80s AD SLE. The crown race measures 26.9 . The JIS headset races
just swim around and the European sized ones just look too small with their 26.4 size. Anyway, ideas on
how to approach this? I've read a bit suggesting that running things loose is pretty unsatisfactory.
The head tube inside diameter measures 29.9 which suggests I should be looking at JIS. Half a mm seems
a bit too much stretch to mix the two sizes.

You could get some material brazed onto the crown seat then re-cut it to JIS standards.

I've never heard of problems mixing a 26.4 crown race with JIS parts but I've never done it myself. If you could find 2 headsets of the same style for the 2 different specs (I guess you'd have to if you wanted to try this) you could just compare the crown races to see if they differ in any way besides the ID diameter. Sounds like a PITA though.

Or you could punch some notches into the crown race seat to see if you can tighten up the fit. If it doesn't work you wouldn't be out much since if you either braze on material or cut it down to 26.4, the notches wouldn't matter anymore.

Using a 26.4 ring just won't work without modification.

After posting this I read my Barnett's and there it suggests something I read here when searching.
If difference is less then .2 mm knurling the fork crown race base will work. I think I will work on this
method next.