1 600w bulb is good for about 4'x4' coverage. With that many plants you won't have much room to move around in there. What strain(s) are you growing? I have a GL240 tent and will likely do a 4 plant perpetual harvest....I also have a GL145 tent. I have a 600w MH and a 600w HPS (2 of each if I want to switch them out. Also have a T-5 8 bulb and 2 250w veg CFL with reflectors for all. air-cooled for the MH/HPS)
I'd top for 8 heads via nugbuckets mainlining technique, flower them a little earlier than he did, so maybe 55 days of total veg time, but still maintain a slightly shorter awesomly level canopy.