The short answer is that you will have to modify everything!
The way that I went was swapping the last three inches of the bell housing, swapping the ends of the transmission input shafts, modifying the drive shaft, linkage bars, and trans mount, and using a mix of W201 and W123 parts putting it back together. Add a number of machinist and welding steps into the process.
Modifying the bellhousings easily is all about the input shaft cover plate, what made this possible was that the plates match pattern and distance wise on SOME of the 5-speeds and SOME of the 4-speeds.
here are a couple threads that go into this in more detail, im still working on both of them, but one is the distilled version of the other and they are from different perspectives.
This is about the project in its entirety (as I struggle along in pursuit of this goal
)-
http://www.superturbodiesel.com/std/617-franken-5-speed-t-323.html
And this is the how to do thread which is not complete, I have only gotten so far as putting the tranny back together-
http://www.superturbodiesel.com/std/how-adapt-w201-speed-om616-om617-applications-t-965.html