MichaelD,
It may be invasive for an early 230 as Michael has said, but for a late 230 (Nov 66) it's a relative piece of cake with no welding at all or cutting of anything more than the cosmetic plate at the B pillar, and a little drilling. All the relevant holes are present to convert to a 3 point seatbelt on the late 230. I did mine in about half a day from start to finish and whilst I didn't do a perfect job cosmetically, it's perfectly safe (has passed the German TUV and UK MOT (annual safety checks for older vehicles) without a problem) and far safer than a lap belt. My car is a US import.
There's a merc bulletin on how to do this that some kind forum member sent me. PM me for details, but please note that there may be differences between early and late 230s, which is what I think Michael S is getting at.
Moreover, there are different ways of doing this. As a tall fellow I opted to put the reel in the area under the parcel shelves (as shown on the Mercedes bulletin) but others - like Garry's car it seems - have taken a different route. There's no loss of leg room doing it in accordance with the Merc procedure as I recall.
I wouldn't hesitate to make the switch for the obvious safety benefit. Do a search and you'll find a good deal of info. Read all that first.
JH