Get a flat plate (strip) of alloy or steel, just as wide as the licence plate and drill 2 holes (centred) so they pick up on the original licence plated pick-ups on the bumper.
Then drill the holes through the same strip of material, so that they pick up on the Licence Plate holes, you can lift the plate a little or a lot, to get it vertically central on the bumper too.
Add a spacer between the licence plate and the material, this will allow you clearance between the strip and it's original bolt heads that attach it to the car (may need a bolt a bit longer - same as spacer height).
Add a spacer between the car and the material, this will allow you to slip some auto 'clip nuts' over the material that you can use to attach the licence plate to this material.
I have done this on my R33 on both ends of it.
A perfectly central front, which sits only about 10mm off the front of the bumper, you can not tell.
And a full standard licence plate that sits (with clearance) in the rear slot. Yes, I also cut those ugly Jap style extensions on the rear mount.
It works well and I am sure that all of you could do it...