If you know it is not the other gaskets mentioned to look at....
Why are you guys skeptical that it is the rear main gasket ?
That is one of the hardest working gaskets in the engine... like James Brown... the suggestion I made... if it is not the rear main remains a good one... since it is also covered by the trans bellhousing...

But I am betting on the rear main because while I know about the oil passage due to reading the FSM... I have never heard of anyone having a leak there..

It is a huge job to replace the rear main seal... so brace yourself for sticker shock unless you are willing to do it yourself...
Almost entirely LABOR...

Why don't you just drive to a few places close by and get estimates ?
Then come back to here for comments on the reasonableness of them...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

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