I'm new at the four cylinder end of this, but I've scattered a TON of W123 five cylinders. I'm crossbreeding a '78 and an '82 240d right now....and both the dead and live 240 motors happen to be resting on the shop floor next to an '81 five cylinder. Mounts are the same (measured/compared a parts car).....as the motors are installed in the cars, it's the most easily imagined as adding another cylinder to the REAR of the motor. Driveshaft and shift linkage rod(s) have got to be shorter on the five-cylinder car....radiator and oil cooler WILL bolt in....(done that) and the wiring will be the same except for the glow plug controller and wiring. Shouldn't be a bad job with both cars setting there....but you'll lay under both of them for a while.

Jim Steere
Everything Automotive
Crystal, MI