I also ran a single 600 in a cabinet.. here I did use a sealed air cooled reflector. Best option for you would be to use similar reflectors, tie them together using flex duct, and pull your outside air through, exhausting it outside; this would be a sealed from the grow room and odor wouldn't be a problem. Pretty much like my cabinet, just on a little larger scale. Do you have places where you can penetrate the wall/ceiling and accomplish this?

This, with the ventilation I mentioned earlier will be your best option.

I'm sure if your motivated you could go find a formula to tell you specifically how many btu's each light would generate and mathematically find out how quickly you'll heat up the space.. but honestly, unless you intend to go to a lower power light, it doesn't matter. Also, using switchable ballasts, you'd already have it covered...