Either machine components or a spacer. Thousands of race cars use spacers for their intended purpose. As long as you have plenty of lug nut thread you should be ok.
The rule of thumb for minimum thread engagement is 1 1/2 times the thread diameter or OD of the bolt....
So if you have a 1/2" thread:
You take 1/2" + 1/4" (half the diameter) = 3/4" of thread engagement...
So if you have a 1/2" bolt, then you need at least 3/4" of thread depth for it to hold well...
Plus the first two or three threads on a bolt are not for holding load, they are tapered... So the third thread is the first good one that you can count on.... The first two at the tip, not so much...
