There are also differences between functionality and reliability. And there are some exceptions in the diesels as far as transmission compatibility, notably the '84, '85 transmissions, which are not as long lived as the previous transmission.
My '79 300TD was retrofitted to use a 300SD trans, the six bolt pan style that is ultra reliable, coupled to an SD 3.07 differential (that was a bit trying however!) I recall the tail piece (yoke) on the transmission had to be changed to fit the drive shaft. Stuffing an '85 engine in it was a lot of work but now its running and super strong.
Its too bad we don't have a lot of 5-speed transmissions here in the states, or I'd swap one into my '87 300D turbo (would that ever make it fly!?!) There is a W124 on eBay with a 5-speed. Too bad its located in Europe O/W I'd buy it (its cheap at that price but the cost to ship it over would make it prohibitive unless one owned a shipping line and lived on the East coast!) see:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=9855&item=2475052924&rd=1
Oh well, I can dream.
DDH