Willem so that measurement 12th fret to E saddle of 321, is that to the saddle wound all the way forward ?
Sounds similar to my measurements and I'm pretty sure we need to chop out a small notch out of the neck or you won't be able to intonate it. May need to shorten the heel as well. Ideally the E string should be about 315 to 318mm from centre of 12th fret, but you won't find this out till it's intonated.
The only other option is to plug and re-drill the bridge holes and bring them forward, must be a common problem with the ash kits.