The worst thing I'm thinking is the transmission interface since 300D motors are automatic. I doubt the flywheel would replace the torque converter easily. Second, I dont think the manual transmisson from my 240D would fit on the 300D motor. Next I am thinking vacuum lines and wiring harness.
Next I am thinking motor mounts, physical positioning.
Last is the AC interface.
Has anyone done this. I would just like to get some pep into my 240D...