I have a mostly complete kit for the conversion of a 123 617 to manual transmission. I ran this setup in my car until switching to a 5 speed. It worked well.
The kit includes
Gearbox from a 240d that works well.
shortened and completely rebuilt driveshaft minus the flex discs. Has a new center bearing. The New universal joint has zerk fitting for grease.
240d 4 speed w123 shifter assembly.
300d Flywheel 38lb
I have to verify that I have the all of shift rods or not.
I put a maximum of 5000 miles on this kit before going for the 5 speed setup. It worked well. I don't recall any vibration issues.
$ 1200 for all. The driveshaft cost me $500 plus the discs once it was done.
Pics coming

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Chris
84 280sl
82 300d euro