if you found this car in boyes hot springs i know this car. white w115 with trashed/ missing interior.
i believe the engine is from a w116, i dont remember it having any emission
controls.
the only engine/drivetrain parts out of the the 240 you want are the clutch assembly, the throttle controls and the rear half of the driveshaft. you will want to trade out the differenial. if you do put it into a w123 and you want p/s and a/c, you will need to change those out and replace the oil filter to its original spot.
that car was set up as a racer and i think got a lot of use. the clutch is worn down. i'm guessing the steel contraption on the passenger firewall with a schrader valve is a propane/ nitro injector.
because the engine oil filter has already been adapted to a 115, you might want to look for a good body w114,115. it would be a better car.
to remove the engine/tranny from this car dont pull it. undo the subframe (4 bolts) and lift the body up.
the men who built that car knew whst they were doing and were excellent craftmen.

don

a 300dt is a wagon