most 400 watt lights put out about 50000 initial lumens there is some grow bulbs that put out a little more than that. and it would be easier to cool one bulb in one air cooled reflector than all those small bulbs put together.
the small hps are not used much because they do not put out very many lumens you can buy a cfl with about equal lumen output as a 50 watt hps,
you shouldnt have much problem with a 400 in a cool tube or other air cooled reflector, you should be able to get the light about 6 inches from the tops of the plants the 12 " and even further distances say for even larger lights is more or less a guideline for open reflectors that are not being cooled.
you should put up mylar or something comparable on the walls to reflect the light back to the plants and prevent wasting any light.