Just been fitting steering head bearings to the CJ250T (I know it's not the 400 but very similar!) and had the same issue as on the 400 where the tapered bearings reduce the clear thread on the adjustment nut.
These are JMT bearings from Wemoto, on the 400 I had the same issue with All Balls ones.
After the 400 problems and having the old original bearings to hand I took time to measure them and the bottom one with washer supplied is exactly the same height, the top one comes out around 0.9mm higher.
Once fitted this allows about 2 to 2.5 full turns of the adjustment nut and I wondered if this is enough to be safe or if I need to do further work to get more hold on the thread.
Cheers,
Andy