A little data and correction to some of the things said in this thread.

In the 86MY there were about 750 US 300E's with 5 speeds. In the 88MY there were about 300. The transmissions were different in 86 than the 88 models. Reverse was in a different location.

Even though the transmissions were different, I would expect that you could interchange them between the two model years as long as you had the corresponding shifter.

Since the M103 was used in some 201 cars, you should be able to use a 5 speed from a 201 with M103 engine. It should bolt up okay, but that will leave plenty of problems to solve such as driveshaft length and clutch master cylinder.

I personally would NOT do such a transplant unless I had the entire donor car that had escaped any previous salvaging, ready at hand for getting the zillions of little pieces off of that I would need to do this in such a way that it would not end up being a butcher job.

I have given thought to selling my 5 speed 300E. I rarely drive it anymore, especially since I now work at home. If I could get a premium price for it I might consider parting with it, but I really can't bear to see it go. The manual transmission 300E IMHO is an absolute gem. The 300E was a GREAT car with any transmission, but when you put a manual into it, you make a car that is just beyond description.

Good luck,