There's one possible solution to this problem that is solid. But it involves using bits of a threaded and threadless headset and a quill stem. Basically, if your threaded fork is a bit too long you can use the top pieces of a threadless headset, add whatever spacers you need above, and then use the upper bearing race and locknut from a threaded headset to keep the bearing adjustment. The stick a quill stem in the fork.

I've got a fixed gear setup this way that I've put thousands of miles on without issue. I can take a picture if desired.