It also looks like you could put a large fan between the PSU's on the front though it would have to be a hell of a high pressure high cfm fan to supply the three 120-140's in the center so open air may be better.
Those PSU's should exhale onto the GPUs or have a vent to push the air above or below the gpu's
All of those thing can be done, or at least most of them. If you are using 120mm server fans they flow about 100cfm each! The center fans can pull air from the front or thru vent slots on the sides.
With everything closed up all the air will pass thru the GPU's and slots between the GPU's.
The front and rear can be turned into swiss cheese to get air in as well.
Do you really need more then 300cfm? I'm running 6 290x's with a 1600w + 850w power supplies and have no problems with temps.
A 5 min phone call can solve all the issues.