Just a quick update that there’s really not much at all necessary to make the GTX 970 work with Yosemite.
Install the card, and make sure it’s set as the primary gpu in your bios.
Reboot, and go to “Options” on the clover menu, and add the following to the end of your existing boot flags/options:
This should enable the system to boot, but with very broken graphics, no acceleration, etc. Don’t worry.
Go to Nvidia’s website and search for drivers for a GTX 600 Series (GTX 680 works) and choose “Yosemite 10.10.1” under operating system. Download that package and install it, reboot without the nv_disable=1 flag and enjoy!