[toastybiz]

400w is perfect for that size footprint. If you are thinking of building the box, you should first take a look at a pre-fab closet cab, try at Home Depot the brand Mills Pride.

The right fan will be a function of heat management as much as airflow. For airflow you want to look at the fan's cfm (cubic ft per minute) rating and have at least three times the volume of the grow space -- in your case you have 40 cu ft so you want a fan rated at least 120 cfm. That may not manage heat, though, so in general I recommend the strongest exhaust fan you can reasonably fit in to your op.

You have decent height but you still may want to use a little LST to help the plants take full advantafe of the light coverage without gaining too much height.

No way to predict yield -- I would say a newbie did a pretty good job if s/he gets 1 oz or so per plant on the first grow (indoor soil). With experience and advanced techniques you could get many oz per plant.