When I look at WIS, it lists removing the portion wall only for the 251 (R-Class). Also, it seems like it is possible to do the job by removing the air box on one side, and a heat shield and computer box on the other (probably also need to be very dexterous).
https://mbworld.org/forums/m-class-w166/632475-60k-service-w-spark-plugs.html