Sounds like your making an oven
I personally think thats way too big for a 21 inch grow box. and if the grow box is only 21 inches tall u still gotta have 6-10 inches space between light and plant so u can only grow the plant to maybe 10 inches. Usually people use cfls for a small grow box like that. Also thats a waste of a 400w hps if u only gonna have 1 plant under it. There made for a area maybe 3'x3' maybe 4'x4' . My answer to yur question is No I don't think it will be possible. Maybe im wrong and you can prove me wrong? Good Luck and Happy Growing!