Build a mock-up out of cardboard and tape. Put some pots in there, and try to picture your lights and plants all together. Headroom goes quick. I wouldn't try to put 2 chambers in there, you just won't have the room (assuming 3x3x4 dimensions). You'll need a decent bit of vert for a full veg cycle, I'd say about 2' with some crafty CFL placement. Before you know it, your plants will outgrow your veg box and your flowering plants will still be a week away from harvest.
Looks like you're also trying to put a lot of wattage in a small box, you'll want an exhaust fan with at least double the CFMs of your ³ft. Home Depot has some bathroom vent fans that move 70cfm's and would be perfect for your box, but they're a little noisy.
8oz dry seems like a lot to pull out of a box that small, but I'm sure somebody's done it. Looks to me like you're expecting 1/2 gram per watt? That's tougher to do with CFL's as they aren't as efficient as HPS, almost half as much.
My prediction? You'll get 2-3oz dry out of that box on your first run, and that's if everything goes right. With some experience (and more lights) you might get 8oz, but definitely not on your first run (assuming you're new to indoor growing by your questions)
Is this going to be a box that slides in and out of a closet? If you have more room, why not go for 3x3x6 or 4x4x6? 5 sheets of plywood or some PVC and panda film will make you a nice sized closet (I prefer wood, more sturdy, can easily mount an a/c, fans, outlet... or whatever)