In addition to the difficulty mentioned by Novaguy engines 117.982 to 117.985 have different assembly bolt location for trans than 117.986 onwards and only 3-speed auto boxes are available for those engines as there is no manual box for the 450 SL/C as Mercedes has no homologated manual box for the 450 engine. The 4 & 5 speed are not interchangeable with 3-speed auto in the 450 engines due to different bolt location holes on the intermediate plate. But given unlimited resources anything is possible.
