I'm on the same route.
Check
www.w113.org (German), from here you can get all you need for installing a 5 speed Getrag 265/6.
https://w113.eu/Shop/Roadbook%20Shop.html Google translate works well.
The gearbox, Getrag 265/6, can be found att Breakers, many available in Germany.
There is several earlier treads in this matter on this site, but they are a bit out of date.
I have got the gearbox, installation kit and the needed shafts, waiting for a Bell house to keep the original gearbox intact.
So I'm about to start the job.