The Box you have designed is Fine, But what-ever light system you go with you have to have good air exchange or it will not work!!!

You should shoot for 4 or 5 plant's and not worry about being 5' tall and have 4-5 at 2'or 3' tall. If heat is going to be a problem? Don't kid your self? That many CFL will put-off some Heat!!!.... My 2-cent's would Be, Go with the 250w. HPS @ get 4-2by or even just single's T-5 Tubes, But any-way put one vertical in each corner.
Since you are so tall make a sub-floor at 12" from the floor-( so you can recess a CFM-fan in the floor ( vent around the very bottom out-side walles of box with cornis vent's they come 3" @ 6", use the top for a fan oscalating down, and very-top exhaust-out. And I bet your result's will Shock-You!!! THT'SMY2CNT'S. Good Luck!!...Later