FWIW, I had the same problem.
I bought 3mm spacers, but ended up with rubbing issues against my MSA type2 bumper.
my dust caps are pretty giant on my 280z. I looked for other options, and theres a set on fleabay that looks like a different design and even lower profile, I'm curious if that will work (or if they'll even fit.. or maybe they're same size, but they just look shorter in the photo).
annoying thing is, once you go the spacer route, or grinding down the wheel or dust cap, the wheel caps still wont fit