2800CS 5 speed
I just finished a 5 speed conversion on a 1970 2800CS that had the ZF 4 speed box. The 5 speed (Getrag 265/6) was from a E12 528e, also used a 528e bell housing. You will need to get a new slave cylinder as it is different from the ZF.
While "I was in there" I also replaced the rear engine crank seal, clutch, PP, TO bearing, and had the flywheen ground. Use new flywheel bolts.
As far as the driveshaft. I was going to have mine shortened the 3+ inches, but th U-joints are somewhat worn and the driveshaft shop advised against it. Since they are not easily replaced, I elected to source an already shortened driveshaft with rebuilt U-joints. Not cheap, but this way I still have the original shaft and tranny should a future owner want to go back. Good luck as once you are done, it is a sweet conversion.
jjs2800cs