I asked a similar question a while ago because I have an '85 300D that was hit in the rear but has a great motor. I wanted an SD so I asked if everything would switch over. I was told that, if you're only swapping the motor, the only thing you might have to change is the engine support arms, where the motor attaches to the frame. If you're using the engine from the D and the tranny from the SD in the SD body, everything should be fine.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo