I don't see why that wouldn't work, my 400W is in a space smaller than that, but then again the front is open so venting is not a big issue as yours would be if you keep the doors closed. The way that is set up already with the 3 sections, I would seriously consider 2 250W. One MH and one HPS with a separate veg and bud chamber. That way you could harvest twice as often, if that matters to you. It's really nice to be able to harvest and immediately be able to start more plants flowering. However I'm not sure you would get anymore bud overall that way if you use the same amount of space either way. If you take out that middle support keep in mind that might be important for load bearing reasons.