A common observation.
The modular construction system of Mercedes passenger cars (and other carmakers perhaps too) often gives us the wrong imagination that criss-cross replacement of parts or complete components might be an easy task.
Not so for an inline six into a W110 chassis.
For the incorporation of the M180 the front body structure of the W110 was thoroughly modified by the factory. The front cross member is different and the radiator surround also in order to make space for the deviating radiator assembly.
This is not a too easy swap for a shading tree mechanic anymore but can be done of course. The easiest would be to have the complete donor car by hand, which I understood you already have…
My recommendation: don’t do it.
If your W110 200 is really as excellent (body and interior) as you describe, leave the car stock as is.
The 200 engine in your 190 would only be of a partial help since it is not the original engine for your SL and would only reduce the value of your +50 year old car. Your 190 SL is only correct with a 121.921 or 121.928 engine (numbers from my old memories, not absolutely sure here) but nothing else. You cannot turn a 200 engine easily into a 190 SL engine although the 5-bearing engine might deliver a smoother run than the old 3-bearing M121.
My 2 cents: keep the (beautiful) W110 stock and overhaul your old 190 SL engine or get a correct one.
On the other side …
…you could place the M180 in-line six in your 190 …;
there was one (or a few) 220 SL (190 SL 2.2) factory prototype with a 220 SE engine available. This would provide the proper refinement that the 190 probably misses.
Or: get rid of the 190 and go hunt after a Pagoda right away…
But leave the W110 as is.
Achim
(4 MBs, none with too few cylinders)