Don't know if this will help...
Here's a pic of my old r34, you can just make out the edge of the plate cover, these are the plastic ones I pinched when I was working for a dealership and they clip around the plate rather than just sit on top with clips.
Don't know the size of the spacer, but it JUST fit on the edges of the reverse lights with the cover and still sat well inside the bar, looking nice and factory.
I used white slimlines which are exact size as your metallic
I'll try to find a pic I haven't completely covered the plate
PS. I too bought euro plates for my new car only to find they wouldn't fit properly, but I went macguyver on it's ass and fabricated a custom plate bracket, wouldn't look as well on an r34 though unfortunately