Having fans push through multiple sections will really reduce the flow rate more than you probably expect. If you are going for mid-range GPU (R9 270 or 280) i'd suggest you use the higher end cards (R9 290) so that you can have fewer motherboards/CPU/RAM/risers which saves money and hastens ROI. My suggestion for air-flow would be to have 2 shelves of miners with fans and filters on the left side of each level. The air flow would pass across the shelf to the right and exhaust at the same shelf level.
You may need to combine the two hot airflows as you don't really show how your vents will connect to the outside world.
If you fan-force this setup then you will need a substantial opening in the garage to allow the pressure to equalize, ie. leave a window open. Although some sheds are poorly sealed anyway especially around the roofspace depending how it was constructed.
Also re the above comment you can also get windows 7 to work with 6 (or more) cards using modded drivers, it takes some messing around:
https://litecointalk.org/index.php?topic=11761.0Perhaps you did not see the pictures? or maybe theres a problem with photobucket.
I actually already have 4 rigs each with 4 cards.
Two rigs are 1200w 280x rigs,
One rig is an 850w 270x rig,
One rig is 1000w 2x270x and 2x280x.
I have them in normal rigs where all 4 cards are parallel to each other with normal risers. Fans blowing through the cards. But temps are difficult to keep under 75 with this setup. I would prefer my gpu fans not spin ridiculously fast 24/7.
Each shelf will have hole saw holes drilled into it in the same shape as the fans on the card. The video cards will face downward so that the only air sucking into the cards are cool air from the fans below. no recirculating air from the video cards exhaust.
I will also have slots cut to allow cool air to pass over the mobo and power supply so they do not heat soak.
The clear separations in the image i posted divide the airflow between the two shelves.
Also, Regarding the bolded text about combining the airflow and exhausting the air. the air combines at the top shelf. cold air is isolated till it reaches the cards on the top shelf then both mix and exhaust together. It will be directly exhausted out a window. There will be either 1 fan or 2 fans in the window. the intake will be from another opened window on the other side of the garage.