I recommend only using a high wattage light that has been manufactured and tested to UL/CSA specifications and not play with a DIY high wattage light fixture.
I know its not as much fun to buy one than to build it yourself, but consider this; 400 Watt fixtures give off a lot of heat, metal halide bulbs operate at temperatures high enough to set plywood on fire, and ballast starting voltages are often measured in thousands of volts!
You could burn your freakin house down!