I'm just not sure how you're gonna get the clearance from the bulb to plant for effective light distribution. With the closet only two and a half feet wide, you'll have to train the plants directly against the wall. Even running six plants, you'll only have 16" of width, about sixteen inches of depth to train into three foot vertical space.why the advice on horizontal bulbs
Do to lack of foresight, I struggled in keeping three foot tall plants contained in that space. Had I trained everything away from the light to vertical screens right on the walls, I might have had a chance.
If you run horizontal, and you practice defoliation, you should get plenty of light penetration into the canopy. So you should be able to generate 14+" of flowers across the tops of the plants, with your total foot print of just under 12 ft square running two 600s. The training is relatively simple.
I think you will spend much more time training/vegging out plants you try and get into that vertical alignment.
Maybe something more creative is in order though.
Maybe a stadium with trained or transplanted horizontal main stems... hum... that google Sketchup has never worked for me. I don't know how to express this idea coherently...
If you want to do six plants, why not a nice 4x8 tent? Each plant gets a nice two by two space...