On 8 1070s wouldnt 2 750s be right on the edge. I pull 1250 watts from 8 x 1070s running at 65-75 percent power. I run 2 x 850s in my 1070 rigs.
How are you powering the GPUs? What brand PSU are using? It would be nice to use off the shelf equipment, looking at these server power supplies Im not seeing how Ill be able to power the motherboard or other devices in the system.