I actually just did this swap. I have a W123 wagon that had a shot 722.3 trans so I bought a 722.3 SD trans as a temp. solution before I did a manual swap.

It will bolt right up with no hardware issues. The issue you will experience is if that SD transmission has the electronic speedo sensor, as it did in my case. The tailshaft is different between the geared speedos in the W123’s and the electronic ones in the same vintage SD’s. If it was from a 78-80 SD, there may not be an issue. But the 81-85 have IIRC an electronic speedo sensor.

A tailshaft swap could be a solution.

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1982 300TDT
1991 190D
2022 Metris