Its def going to be a size issue, remember the reflector is like 2ftx2ft at least. Plus you need room so it isnt all jammed against the wall and such....dont want any fires. you could do it minus the reflector, just hang the fixture above with reflective material over it. when you expand the grow area, just pop the fixture back into the reflector and wallah. crude, and some light will be lost, but if you make your own reflective surface its def possible, i have seen grow ops with 6 1000 watt fixtures just hangin between the plants with no reflectors at all, use your imagination and plenty of research.
If you can properly vent the lamp with 6 ft of height i think you could go with the 400. Make sure that you vent the heat straight out of the room though otherwise you will have serious heat problems and even safety concerns. If you can't vent it properly out of the room forget about using a 400 hps.
