250w hps heat and headspace?

I want to build a stealth cab and I just scored a free 250w hps light kit, my friend replaces street lights. How much room between the tops of plants and the light do I have to plan for? How much heat will a 250w hps light put out, more than 10 23w cfl's?

Tuna

I have my 250 watt about 5in from the top of my canopy. my temps get to 85 degrees at the highest but that's just because it's summer time and it gets hot out here.

Get yourself a cooltube for it. That is what i did and i can actually get as close as 3in from the tube and my temps are in the mid 70's.