Solution
690. This time around, nvidia should have a much improved dual GPU card on their hands, unlike the 590 which had many problems.
The 690 will also take two slots but I think that you won't have an issue unless the pci-e 3.0 slots are close together. Usually there is a space in between the full length slots to accomodate the two slot cards. You may only end up covering a x1 slot.