Thats 3 foot deep??? Looks smaller to me. So one half is 6' tall x 2' wide x 3' deep? Thats 36 cu ft with a footprint of 6 sq ft - a 400 is way overkill for that cabinet IMHO. Sure you are at 67WPSQFT, but with the heat issues you'll get, theres better solutions. I'd get a bunch of cfls, or a 250, or even better, a 150HPS and 100 MH, build a custom reflector and have a 250W Dual Spectrum over one fat scrogged plant in a 5+ gallon bubbler bucket - but im nutty like that

You can run whatever lights you want. It all depends on your fans and ventilation. A 400W light puts out a lot of heat. If you cannot remove that heat quickly from that tiny grow area, plants will die. You could run 1000 watts in there with proper cooling. Just buy good fans, don't be looking for "inexpensive" ones. Cooltubes work great if you duct them properly, and take intake air from a cool source (NOT inside the growbox).

Where is the cab located in the room? Where will it exhaust and intake from? What are stealth requirements?