H Rob ,
I went all through this a while back with a forum here, probably 18 months ago,
to help you, yes as you said probably use one of the holes and remount,there is no backing plate
on the under side of the later 280 mirrows that i fitted to my 230, but used good size washers,
yes thats correct i did the opposite to you, except the lady that owned my 230 in CA, was 86 years old
and she had these huge big mirrows out on both fenders ,so i removed and fitted later 280 SL to correct position,
and refilled holes and painted my self.
Also i may be wrong here , but the 280SL mirrows i dont think stick out as much as the early goose neck 230SL.
so not as prone to bumping or knocking into.---regards.