The dash will swap without any problems. The 240D had a center lever control for the center vents. The flap can be made to stay open and the horizontal slide lever will not be there. Since the donor car is a 300D the center vents each can be controlled individually.
You will need to drill a hole for the idle control knob.
I would suggest a new rubber tubing from the center vent section on the evaporator box to the 2 center vents. The 300D type rubber section is the only one available. The 240D unit is not made. The 300D center vent box actually sits farther back and does not rotate a full 90 Degrees rowards the center vents as the 240D unit does. So the 300D rubber section will fit on the 240D box easily. You will see this when you remove the dash. Order the rubber vent early, It will have to come from Germany.Go here to see the 300TD and 240D center vent area side by side. Its the 3rd and 4th from the bottom.

http://www.peachparts.com/shopforum/showthread.php?t=156207

Go here to see the differences between the 240D and a 300TD.

http://www.peachparts.com/Wikka/W123Evaporator

A tach swap is difficult as pointed out above.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car