You can remove the engine without removing the transmission. There is no splined shaft with an automatic transmission. You unbolt the transmission and torque converter from the engine then lift.

The engine mount arms obviously need to be swapped but make sure you check whether you can run the SDL oil filter housing and oil cooler lines in the TD. If they have to be swapped, do it while the engines are out of the car. I think you can use the TD oil filter cap in the SDL housing so no need to swap housings just for the cap. It'll be hell removing the SDL oil filter cap in the confines of the TD engine bay.

I don't know how much of the cruise actuator and throttle linkage have to be swapped between the SDL and TD engines. A safe bet is to swap cruise actuators and the throttle linkage bits attached to it.

Sixto
87 300D