Take a look at this
Mercedes-Benz OM617 engine - Wikipedia, the free encyclopedia
Power is different - .951 is more powerful
I guess the .950 when fitted to the W116 was using a 722.1?? transmission...
...and the .951 was using a 722.3?? transmission.
The diesel engined versions of the 722.1 and 722.3 will swap between each engine - no problems. However not all 722.XXX transmissions are good for diesels... => different bell housing for petrol engines especially V8s - and different valve body and shift points. Early 722.118s don't even use vacuum for shifting but use a shifter rod to throttle linkage - whilst later 722.118s only use vacuum... so the message is get a transmission that suits your engine if possible!
What type of transmission is fitted to your N/A OM617 engine at the moment?
The 722.118 isn't well regarded - it suffers from spline shaft wear problems and front pump problems
http://www.peachparts.com/shopforum/diesel-discussion/299729-advice-reqd-shaft-spline-wear-measurements-possible-fix-722-118-transmission.html
I guess - and I'm only guessing - the extra power from the .951 comes from a different cam and a different IP.