Gordon,
Here's a picture of a RHD scuttle from beneath, showing a circular cutout for the antenna. The cutout is centered on approximately 34mm and 60mm, relative to your 30mm and 120mm. These dimensions are a little difficult to measure because the hood/fender/scuttle reference lines are slightly curved, not straight, as I'm sure you know from doing it yourself (are there any straight lines on a pagoda?).
The equivalent dimensions on my LHD 230SL are approx. 32mm and 60mm, and again I think I can feel the circular cutout inside. So perhaps all LHD and RHD are the same in this respect. I'm pretty sure there's a factory document explicitly stating the antenna location and hole size, I even think I have one, but cannot find it. Here's one for the alternative rear antenna location but that's probably not relevant for you.
Bottom line is I think your 120mm should instead be 60mm! Which definitely should be checked & verified before drilling.