I asked the exact same question two months ago and just finished the installation last week. I am absolutely ecstatic with the results and really, really, really like the car with the 4-speed manual. I have no vibration at all unless you really lug it, and you need to know what you're looking at to know that it didn't come from the factory this way.
I documented the installation here:
http://www.peachparts.com/shopforum/diesel-discussion/334272-300d-4-speed-swap-you-happy.html
Including the dimensions of the pilot bearing you'll need to make...
In the end it really wan't a huge deal and I just worked on it when I had time, just enough to keep the project moving. Everything "almost" just bolts up.
Like Charlie said, it'll take a long time for this silly grin to get off my face.
Happy to help in any way.
-Dan
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1976 w115 300D 4-speed conversion!!
1985 w123 300TD 4-speed conversion!!
Non-Mercedes old stuff:
1941 Buick Special (Straight 8!)
1946 Luscombe 8a
1947 Willys CJ2a
1950 Chevrolet 4400 dump truck
1954 Chevrolet 3600 3/4 ton
1954 Cessna 195B
1955 John Deere model 40
1991 Ford Taurus SHO
2013 Jetta SportWagen TDI 6-speed (I sold out...