Maybe I didnt catch all this but you got a 600 watter in a space 3x2x6. Think the 600 will be plenty o light for the space if you can cool it. Is the ballast outside the closet, that helps a shitload. Look I just went from a 250 watt w/ built in ballast to an air cooled 400 in a closet 5x2x8 and holy shit its cooler than the 250 with less fan. I mean its hooked up to a 150 cfm exhausting out of closet with a 100cfm fan at top o closet that pulls air out of closet thats filtered through a can filter. The temps stay in the mid 70's with the aircooled lamp
where as with the 250 not air cooled I had a hard time keeping it lower than 82 degrees. The exaust fans are roughly 80$ for smaller one and 100$ for bigger one. Even if you dont use a cool tube, which mine is not air cooled seemed to work great for me. I can keeep the light hella closer now. Oh yeah and I got a 16 inch oscillatin stand fan in there that runs on low 24/7, it's quiet as hell.

Good Luck