As long as you have some fresh cold air directly
feeding those 970s from the outside, doesn't necessarily have to be a 38mm high rpm fan. I'm using four of them in the upper chamber of my S8S, two mounted on the 240mm flexbay radiator I'm using are necessary to achieve the best performance for that specific type and brand of 60mm thick radiator.
But the other two 38mm fans I'm using as unobstructed clean air intake and rear exhaust could easily be replaced with extremely efficient 25mm fans at medium rpm and still cool the graphics area just as well. (I just think the 38mm fans look so bad-ass)
Positioning of the fans is paramount, left front and right rear will create a natural cross-flow of cold air over your graphics cards, the cold air has no choice but to flow into your PCIe/graphics area on your motherboard and then is directed (pulled by the rear fan) from left to right and front to rear, there are no other options for the air to take. At least no other options in my S8S set up since the top is solid acrylic offering no air flow.
I also have a minor bit of warm flow off the radiator but that should be pulled to the rear without any change in direction mixing with the warm graphics card air and exhausted out the chassis rear.
Let me know your radiator placement and chassis airflow planned thus far. This stuff is not rocket science, it's only hydrodynamics and thermodynamics, and you're in good hands.