The behavior observed by the OP is somewhat typical of the 603 under load, however, the climb past 105C. is definitely extreme.

As mentioned, the thermostatic fan clutch in front of the radiator is a key component in keeping this engine below the red zone. Once the engine hits 100C., the fan should be fully engaged and making quite a bit of racket in moving a significant amount of air.

If the fan clutch checks out, the real question is the age and condition of the radiator. If the radiator is original, it most certainly is a contributor to this behavior.

The suggestion that the head gasket is compromised without any supporting evidence other than higher than desired temperatures under load is simply a WAG that is worth nothing.