Hi Jorrit,
It is my understanding the 600d box is more than strong enough for a hot 903cc, are you using a 127 engine or 850 engine? as 850 motor spins anti-clockwise and 600/127 spins normal clockwise. I am using the 600d box with a 903 at the moment and it is good! I do not know enough about the Multipla box to recommend but think it may be stronger given the application, if anyone knows here Guy will!