IIRC the SEC rides on a shorter wheelbase than the sedans so if the 722.315 or whatever transmission yoke isn't in the same place, sedan parts won't help.

A W123 engine is fine. Trade the transmission for a W126 transmission or just swap the tail housing for one which contains the right speedo drive.

Make sure the gasser fuel level float will survive a Diesel or alternative fuel environment. I don't think a Diesel fuel level float is a direct fit in a gasser tank.

You're not really hacking up an AMG, you're just changing the engine. Do it right and there's no reason it's can't become a gasser again in the future. The conversion should be completely and easily reversible if you keep all the parts and plan and document every step.

Sixto
87 300D