There were 2 or 3 factory W113s with the new 3.5l V-8 made in late 1970/early 1971 as test mules for the upcoming R107.
Back in Germany, in 83 or 84, I saw one that was done by a private owner. There were several around where I lived, that had the twin cam 2.8L. One thing that always bothered me with these, was that the tach did not work.