IMHO you would have to go w/ one from either a 16v or maybe the 129. I believe they both have the getrag tranny and you need all the beefy-ness you can get. Mercedes never put a manual in anything larger than a 300 fo a reason...
-M-

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Currently:
1972 350SL Euro 4spd
1973 BMW R75/5
1981 BMW R80GS
1995 FZJ80 with OM606 Conversion In Progress