I have a 4ft square tent (about 2m high) with a 1kw super hps in there. It's in a small room that has it's door closed 24/7, the radiator is off and the window is open.
To keep the temperature in the tent around 25C or lower I pull air in through a duct that goes to the window. My tent has vents that open on the INSIDE, so if I leave the tent like that, the vents still flap out inside and allow air to get sucked in through that path (because it's easier than getting it all through the inlet duct). Because the room the tent in is closed so often, letting air recirculate through the side vents lowers fresh CO2 concentration and lets the tent warm up quite a bit. I purposefully block the tent's vents from the outside by putting a few sheets of paper over them (which stick because of the slight vacuum in the tent trying to suck them against the vents).
At night, to avoid them getting too cold, I just unzip the door and let it recirculate the air in the room, which keeps them happy. During winter, pulling in fresh air 24/7 through the night became an issue because it was getting down below 0C outside.
With sativas in 3g pots (15l) they're finally flowering and are up near the lamp.
As your tent will be twice as long as mine, I'd go with two 600w's or three 400w's. The 400w's would get my vote. Really, if you can fit the shades / tubes in, you could even go for four 250ws. Each bulb will be producing 1/4 the heat of a single 1kw lamp. Overall, the heat they give off will be the same, but there'll be less under each, so you'll be able to run them closer to the plants and have to worry less about running out of vertical space.
You'll also want to sort out ventilation with ~250 -> 300cfm of air going through for each kw. Cooltubes would be nice, it's necessary with my lamp since so much heat is in one place. If you're hard for cash, you could probably make fine tubes by scoring a length of wide drain pipe then sawing a window into the side and lining the inside with kitchen foil. If you use multiple ballasts, stagger the time they switch on so they don't all clunk on in harmony, which may trip breakers and encourage fires.