if its possible, get your ballast out of the growbox. The ballast almost generates more heat than the bulb, and in small growing spaces heat is an issue. I just have a feeling your gunna have a problem keeping plants short enough so that they wont get burned. You only have 3 ft ceiling and that 400watter needs to be about 2 feet away from tops.

Its probably still gunna get hot as fuck with that light on, run a test run with no plants in there and check the temperatures . Its better to deal with a heat issue when before it kills or stress's your plants.