The three way cats of the 380 will never take the fuel mixtures of the 450. They will overheat and destroy themselves very quickly.
The 380 cats need an O2 sensor Lambda control system to keep the mixtures much leaner than a 450 can run.
The best combination would be to get an exhaust from a 77 to 79 450. This would have the cat under the car. It also would fit. The 4.5 is a wider motor than the 380 so I doubt that the crossover cat will be long enough.
Bosch Master, ASE Master, L1
33 years MB technician