Guys...

I did this about 2 years ago...at a total cost of around $700!!! Call around salvage yards and locate a W140 4.2 119 (400SEL, 400SE, S420). It will most likely have ASR (mine had asr, which worked out well). I got mine for about 550. When you take the old one out, all you have to do is swap out the back plates, you will know what i mean when the time comes. I swapped the diff's out in under 2 hours myself, in my garage. Its not that hard.

Regarding the speedo calibration, dont touch the instrument cluster. You dont need to change any gears out. The speedo is electronic. There is no cable. It uses an electronic pickup on the tranny. Buy this:

http://www.abbott-tach.com/era.htm

Its about $125. It converts it electonically. Its easy to hook up (behind radio) and works perfect. And if you ever switch back, simply uninstall it. You can also run custom tire sizes and dial in the correct ratio to make the speedo accurate.

Let me know if you have any questions...

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K