Thanks for mentioning me, I am doing alot of experimenting in flouros. They are very space saving (could stack cabinets), and the low heat means alot to me (desert). I don't think you need 1000 watts worth, but in a box that size, I think flouros would work well to maximize plant height.
If you can't make the box 4 foot in 1 horizontal direction, then you could pickup some compact 125w fixtues, but if you can fit them, 4 foot shoplights are cheaper.
Another bonus of flouros over hid in a box that small is it is very easy to mix the bulbs and get a wider spectrum.
I could go on and on about overdriving, bulbs, heat. Let us know if this sounds like a good plan and how much ventilation, $ for lights you can put in there and we will help ya figure the rest.
'edit' I'm high relaxing, had to reread your post above. I have a 2x4 cabinet, 6' high and sometimes run 4 overdriven (remote ballast) 4 foot T12 bulbs mixed with daylight (6500k) and cool white (3100k). At 128 each (512 total) watts I should be hitting close to 24k lumens, with no where near as much heat as even a small hid. Plants have touched the bulbs with overnight growth with no burn. In my grow journal you can see they are very green, 9 leaves, 1/2 inch nodes etc.