the 250s is a carburated straight 6 in the w108. this engine was found in the w(114-)115 which was the mercedes diesel model from 1967 -1976 or so, so a diesel engine of this era would fit without much trouble.

i have thought seriously about putting a turbo diesel in a w115.
the replies i get from inquires is the engine bay doesnt have
quite enough room in the right places and some minor mods
would be required. the w108 engine compartment is larger,
so i would bet sizing isnt a problem.

mb was pretty good about making engine mounting the same
through its 4-5-6 cylinder engines.

sounds like a cool and workable project

don