First off, I have two systems with four video cards in them just as tight without an airflow problem. I operate at positive pressure, so air is always flowing out of the case.
Second, your video card is designed to work just fine at x8 without a loss of power (graphics power). So if you must use that slot, then you are safe to do so. In fact, you may actually see an increase of 1-2 fps by doing so.
So, your choice as to which way to go.