stackheight is too large (threaded headset)

Though it only uses about 0.5 round of thread now, it does 'lock' the race, but I'm afraid the thread will break.

Is there another solution without buying a new headset?

Possibly silly question: have you properly tightened this all up, or just assembled it hand-tight?

I has similar issues recently trying to get the right number of spacers for a new headset. When I assembled it, the locknut only seemed to catch about half a turn's thread. But when I actually tightened it up with some spanners it went on 3 turns, which was plenty. This may have been partly due to the headset cups settling slightly and everything straightening up.