Not getting video from GTX 970, Mac Pro 2008 (MacPro3,1)
This was solved by:


making sure the PCIe cables were properly seated, on the Mac Pro motherboard.
clearing NVRAM by cmd + option + p + r.
installing Nvidia drivers (which adds the nvda_drv=1 flag as part of the installer).
place GTX 970 in slot 2, making sure PCIe connection is on tight.
system boots properly.
shutdown, then move card to slot 1, making sure PCIe power connection is on tight.
system boots properly.