That would be perfect to grow sea of green 6 plants. Put in a shelf at about 50 inches and use the top for starting plants and cloning under a few cfl, spiral flouros, hang your 400watt HPS under the shelf and grow 6 plants at a time. the bottom is used for flowering. You'll have to grow short plants like no more thas 30 inches, but after 6 mos. you'll have a continuous supply of bud! An oscillating fan about 6 inches in dia. should keep the plants cool , you may have to get a separate fan to cool the light. Actually a 250 watt Hps would work in this space, but if you already have the 400W, use it, up to a point, more light is better!wow... sorry.. I forgot, I have a little closet with 31 inches per 16 inches per 67 inches (80cm per 40cm per 1,70cm) to... Do you think thats will be more apropriated...? And about the light and ventilation for this space, what I need?