I'd be more worried about the 6950's length :P
That case is not eATX, so the amount of space left will be VERY small (the back of the card) towards the disk internal bays.
Try to manage the cables in first instance. That's from experience. It won't matter the amount of vents you put in it if there's no room for the air to flow.
Now, I don't know by looking at the picture of the card if it will blow all the hot air out or not; it appears that it won't, so I double recommend doing the cable checking for a nice airflow.