You don't say what year your SD is. My son and I have converted three 300D turbo Diesels from automatic trannys to 4 speed manuals. Since the autos won't do anything until the rpm is up to 2000 and the turbos kick in, you're sol. However, with a manual 4 speed you can rev the engine if you're of a mind and pop the clutch and get a move on. If the pedal assembly mounting in the SD is the same as the pedal assembly mount in the 240D, then the whole swap is possible and modular. 300Ds become a totally different car!!!!!!!
Ben