I would highly suggest you don't try and run all your risers and GPU's on one rail...I must be misunderstanding the post above? I dont know about the 1070's yet, but my 1080's use 180 watts each. Putting just two of them on one rail would be overloaded.
Card 1 riser - rail 1, card 1 gpu - rail 1
NOT Card 1 riser - rail 1, card 1 gpu - rail 3 or some other combo.
Each 12v rail has regulation. Each card is pulling on the rail. Don't be a dumb ass and put 4 cards on rail 1 and 4 risers on rail 2. Keep them together if possible. Overloaded rails cause regulation issues is my point.
GPU's pull a lot of power when mining begins. It can cause the 12v rail to pull down then pump up...imagine a ball bouncing. This is very true when the tdp hasn't been turned down.