Here's the Cable I need

You'll need two of these mini six pin to six pin cables.

http://www.amazon.com/gp/aw/d/B00G2UDJIW/ref=mp_s_a_1_1?qid=1451855286&sr=8-1&pi=SX200_QL40&keywords=mac+pro+pci+power&dpPl=1&dpID=41Vi7b5r1DL&ref=plSrch

The mini end plugs into the motherboard under the first two hard drive slots.

Source: I have two GTX 660s in my 3,1.

Here's the cable I need