Whats wrong with PCIe slots #1 & #3 (#3 being the third red slot)? The space between them is even larger, which shouldn't be bad for cooling. Besides, using x8 PCIe lanes hardly impacts performance at all. Especially if you use Gen3 cards...
If you havn't read this already, take a look at: http://www.techpowerup.com/reviews/I...press_Scaling/

Cheers