I put HIF6's on my 122 and have been happy with the results. They will fit right onto the same manifold a the HS6's. Make sure you get the heat shields. As I remember, the linkage took a little bit of fiddling, but nothing extensive. The throttle shafts on the HIF6 carbs don't wear the way that they do on HS6's because the return spring doesn't pull them against one side of the bore. The bimetallic strip lowers the jets when cold to enrich the mixture.
One problem that I ran into was that the choke cable for the front carb wouldn't quite reach because the connection was on the front side of the front carb rather than the rear side like the HS6. Again, it's been long enough ago that I don't remember what I ended up doing to resolve the issue.
--
1966 122s, 1968 142s, 1969 144s, 1979 245dl, 1989 244gl