the footprint for a 400w h.i.d. is 4x4'. 2 would work great.
fluoros work well as stated above, but you already have 2 400w so go for it. Just make sure that you have good exhaust to keep a healthy airflow; And a good fan across top of canopy.

footprints by size.
250w 2x3'
400w-4x4'
600w-6x6'
1000w-8x8'