I've got a friend who will sell me a '72 220d with a 4 speed for a very reasonable price. It's got a good engine and transmission, but not in great shape otherwise.
Would that engine and transmission swap easily into an '82 300d ?

It would but it would be completely gutless.

technically you can, but its not really practcal

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Would not be a good result in the end...
but check out the thread on using a diesel for running a generator or welder....it would be super for that... and you can sell the four speed if you want to...
If you can deal with the no power... maybe just find a good body to put that into...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html