I have that same tent in 39" x 39" x 72" with a 600 HPS and heat buildup is an issue even with a air cooled hood and a 220 cfm running on full.
The limiting factor here is the heat. If you use a sealed hood with air venting to the outside like the pictures above, and keep the room cool, you might be able to get away with a 400. I'm thinking a 250 watt may be the biggest you can use in that tent without heat stressing your plants.