Since the W123 and W126 (with the OM617.xxx engine) both use a 722.xxx auto transmission, IMHO there's nothing significant to keep you from executing the same auto-> manual conversion on a SD. I'd be willing to bet that the W123 clutch/brake pedal assembly will alao fit in a W126. And, I wouldn't be surprised to find support cutouts already in the body for the manual transmission's rear crossmember/support.

But, it won't quite be a cake walk; there are a few issues you'll need to address:

1) The SD transmission has an electric speedo sending unit so you'll have to figure out how to mechanically drive the SD's speedometer (it might be possible to swipe the sender off of the old auto and adapt it to the manual replacement or, swap out the guts of a W123 speedo and install it in the W126 cluster)

2) Manual W123s use the brake fluid reservoir to supply the clutch master cylinder. Does the SD's brake reservoir have the same blind 'tit' to facilitate that connection?

3) You will defintitely have to fabricate a rear crossmember to support the back of the transmission.

4) The shifter may not be a drop-in and might require some modifications to the SD's body or the shifter to fit and work (just a guess)

5) Shifter rods will need to be lengthened/shortened. but, you have to do this on a W123 also

6) Driveshaft will need to be lenghtened.

7) An exhaust hanger will need to be fabricated.

8) The neutral safety/reverse light switch wiring from the auto tranny will need to be adapted to the manual shifter's interlocks.

If it were me, I wouldn't be doing all this on a W126; I'd do it to a W116...a much better looking car IMHO.

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Never a dull moment at Berry Hill Farm.