FSM job # 01-418 says 0.50mm can be shaved, and 0.016" is approx 0.41mm so that should be ok. Max unevenness is 0.08mm (0.003") in longitudinal direction only. The prechamber protrusion will need to be measured since 0.4mm was shaved off, if they stick out too far, they will need to be shimmed upward to compensate.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/01-418.pdf