yeah you will def need some strong fans plus cool tube or cool hood to keep that nice box closed with the 400hps. I have a box measures about 2x4x7 and I had and still have heat problems. Now I have (1)30w intake (1)250w exhaust (1)30w fan for the home made cool tube and a small fan blowing the plants and I still have slight heat probs my temps are about 87-92 with my door completely shut. My temp used to be 110 with the door shut and just one big fan blowing the plants. Dont worry not everyone has the same problems I did it also depends what kind ot room temp you have and a window would do wonders. Why dont you do a test run without the plants or I'm guessing you dont have the lights yet?