Ted,
If you plan to show the car, then don't put on the guards. Some of the US-spec 230 SLs came with awful huge bumper guards, but most did not. If you've seen the prettier chromed 280 SL guards on a 230 SL, then it was added later and, technically speaking, would not be "correct."
Run a search on bumper guards in the forums. You'll see pix of both examples.