I seriously doubt one of those 240mm rads will handle 400w very well..
Do you have any figures to back it up?
I am just going by my 360mm heater core radiator performance, I have dumped 1KW on it by watercooling a condensor before without problems. It is a matter of having the right water and airflow. I had 4 fans in pushpull so I was just using 240mm of the radiator and a 1000L/h pump and was just about 15ºC over ambient....