Its not a hard process, you need a W123, manual transmissions into W126 is very hard.

Search for a thread started by Rleo, entitled manual transmission swap has begun.

Its well documented, the difficulty is finding the manual out of a 240D.
Also you must live witha 4spd manual, a 5spd is very very rare, and expensive, I personally have never seen one under $1000.

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1984 Euro 300SDC, (4spd standard)
1986 Toyota Landcruiser Diesel HJ60 5spd X2

Gone but not forgotten (some sold, some stripped)
1983 300 SD, 1985 300 SD, 1983 240D, 1986 300 SDL, 1985 300 SDL, 1983 300 D, 1984 300 D, 1985 300SD, 1987 300 SDL, 1983 300 SD, 1985 300 TD Euro, 1983 380SEC, 1990 300 D, 1987 300D, 1982 300D, 1982 300D, 1994 E420, 1987 300 TD, 1987 300 D, 1984 300 D