The 2 sixes would be nice in there, but if you got the 2 400w's all ready then you should be good to go with a SOG applacation for the fact that you just drop them as close as you can an basicly this way you have a 1.5x3 for each light to cover wich will be cool coverage with the short plants, but if you have future plans to grow any bushes then you may want to plan a head and think about those 2 sixes. I think 10 to 14G per plant is possible with not to much work. Even more is possible depending on the strain.
Adieu