Warden,

Yes, 5 speeds were found on many of the European 123 Model diesels, I don't know about the Sd though. Some of the Mercedes 5 speeds came with 5th gear as an overdrive and some with 5th gear being one to one, meaning that you had a close gear ration which should mean snapper acceleration.

The only difference from the 4 speed installation, is that you'd have to obtain the shift knob assembly from inside the car and the linkage from under the car to make certain that everything worked smoothly. As an example, when fitting a 4 speed to a 300D turbo, one has to add a couple of inches to the driveshaft and have it rebalanced. The 4 speed tranny is slightly shorter. we don't know yet the length of the 5 speed but it almost certainly is longer than the 4 speed. There were apparently four different 5 speeds for the 123 Models. Some apparently had aluminum bodies instead of cast iron. I've located five, 5 speeds with shifters and linkage and are getting ready to bring one in for installation, are you possibly interested in one?
Ben