There are reports of GTX 1070 FE working in Mac Pro 5,1. Personally running a GTX 1080 FE without issues in a 5,1.
If you're going with one of those cards (or one with an 8-pin power requirement), you'll need a dual mini 6-pin to standard 8-pin cable to power. Many available, but know for sure this one works:
https://www.moddiy.com/products/App...tandard-PCI%2dE-8%2dPin-Video-Card-Cable.html
Sticky thread has all setup & install info.
If you're going with one of those cards (or one with an 8-pin power requirement), you'll need a dual mini 6-pin to standard 8-pin cable to power. Many available, but know for sure this one works:
https://www.moddiy.com/products/App...tandard-PCI%2dE-8%2dPin-Video-Card-Cable.html
Sticky thread has all setup & install info.