You can use a 400 in that space if you have enough ventilation. Have you considered how you're going to exhaust the heat? Will you use an air-cooled hood? You have to calculate these and probably more details into your decision.

You can get better results with fluorescents in smaller spaces. Fuorescents don't need much space above the plants like HID lights. There's less heat problems too. T-8 flourescents are good for growing and are cheap.