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=11761.0