Probably not that difficult to get it running but there's considerable work getting the ELR hooked up. All that buys you is idle stabilization. There's an idle adjustment screw on the 603.96x IP so you can set it at an acceptable idle and forget the electronics. You can forget the ADS bits since there's no emissions test for Diesels in California.

The throttle linkage might be another area of difference.

The engine cooling lines will need some work since the SDL oil cooler is under the left headlight, not attached to the radiator. I don't know the oil cooler layout of a D/TD.

I"m not sure if an 84 SD tach will take the pulse input of a 603. I've read that there was a change in tach amps in 84 or 85. Whenever MB went from a crank damper sensor to a flywheel sensor.

The 603 transmissions have a complicated vacuum control system. My SDL doesn't shift properly when I bypass the vacuum switching valves and amplifiers and run only off the proportioning valve on the IP.

I don't think the prop shaft will be a problem. It seems that the trannies are the same length so the SD can keep its prop shaft. It's probably more work cutting an SDL prop shaft or extending a D/TD prop shaft anyway. The fuel input and return lines are in about the same location in the SD and SDL engine bays. The lines to be of the same diameter.

Sixto
91 300SE
87 300SDL
83 300SD