Well you can buy a mogul socket and wire it to the ballast. Then you should be able to run 400w mogul based metal halide bulbs.

http://www.marinedepot.com/ps_ViewIt...s~vendor~.html

If your a little iffy whether or not it will fire a bulb, you can find a used 400w mogul based bulb on here or online. That would save you some money just incase the ballast is no good.