Hi Kim! If no one else has, I'd like to welcome you to reefs.org.
The issue of heat generation is considerable, but since you've opted for VHO and not metal halides it won't quite be as bad.
When I built my own hood I was cheap and used NO fluoros only. I set them about 8" high, heat was an issue because I decided to house the ballasts in the hood itself, so part of what I did to deal with the heat was to leave the back portion of the hood open.
You just want to make sure that the bulbs are not touching and that you allow yourself enough room to clean and replace them easily. Are they the standard 4' long bulbs? If so, then I would set them in something like a stair-step fashion, but you could set them in a more end-to-end fashion--this would leave areas of lower lighting and some folks prefer this. However, IIRC, a 75gal is usually 4' long, and then you'd just place the bulbs parallel to each other. I also suggest staggering addition of new bulbs so that when they have lived out their useful life you don't have to replace them all at once (this can indeed cause burns to animals accustomed to living under a lower light intensity). Kinda like tires, yeah?
If you're worried about heat buildup I'm a big fan of muffin fans. (That sounded really silly, didn't it?) Place one on each end, one pushing air in, one pushing air out. This will benefit gas exchange, but will also really increase evaporation.
