I've been struggling with how to maintain the electric speedometer in the 380SL and none of the early solutions involving swapping tailcones panned out.
But I figured out how to move the internal VR trigger wheel to the driveshaft and keep the tailcone with the manual speedo on the 722.4. Here's a diagram of the strategy. (The full thread on this topic, including images of the proof-of-concept test is here -
Mechanical to Electronic Speedometer Conversion )
Essentially the driveshaft is turned into a trigger wheel by permanently affixing properly shaped and balanced "teeth" to the end nearest the transmission. An excellent mounting point for the sensor bracket is the rear transmission mount. Everything else remains stock. In the graphic above, the purple is the stock sensor, the green are the "teeth" and the red is the custom mounting bracket.
This strategy will likely be a good solution for others doing manual transmission conversions in cars with electronic speedometers.