Spacers, particularly at that height, are a thing of threadless forks & headsets. I don't think I've ever seen such a spacer stack on a threaded fork. Maybe 2-4 mm worth of washers to allow for future headset replacements.

Also, while you can fit spacers between top race and lockring, the top race has to thread all the way down to the head tube.
Will crown race + bearing balls + lower cup + upper cup + bearing balls really eat up 18 mm?