No, W123 grille is narrower - there is no interchangeability. 30+ years as an M-B Parts Manager.
www.w116.org/ should be a help.
1975 280 SE w116 engine number can be seen in between the engine block and distrubuter. It can be covered with dirt and it may be hard to see. To reach and clean it you should open the distributer cap.
The fuel pump relay for the 1978 Mercedes-Benz 280 SE (W116) is typically located in the engine compartment, near the battery or along the inner fender. It can also be found in the relay box, which is usually on the driver's side. For exact positioning, it's best to consult the vehicle's service manual or check the relay diagram on the cover of the relay box.
I believe i may have found the answer in the user manual for the W116 at the following site: http://handbook.w116.org/Maintenance/MY72/120.pdf Good luck!
Latitude: N43° 36' (43.623187) Longitude: W116° 13' (-116.320325)
The Mercedes-Benz 250 C and 300 C are classic models from the W114 and W116 series, respectively, produced in the late 1960s and early 1970s. The 250 C, known for its sporty coupe design, features a 2.5-liter inline-six engine, while the 300 C, part of the luxury segment, is equipped with a more powerful 3.0-liter inline-six engine. Both models showcase the brand's commitment to quality and engineering excellence, with the 300 C typically offering more advanced luxury features and performance capabilities compared to the 250 C. Collectively, they represent a blend of style, performance, and prestige in the Mercedes-Benz lineup.