Yes, both types of M-100 engines (6.3 and 6.9) have been put into R107 cars. I don't know that an M120 has, but certainly it's theoretically possible. Other conversions have been done as well ... a 6.3 into a W113 280SL, a couple of different M120 projects going into W126 coupes, M119s into W126es, etc.

Biggest problem with dropping an M120 into any car is the electrics/electronics, and crafting the wiring harnesses.

Cheers,
Gerry