I'm planning to make stacks of 12 boards. I've chosen a OCZ ZX850W PSU. It's a single rail with max. 70A at +12V. It has 4 PCI-e connectors, so I can connect to 4 boards. From each of this boards I'll daisy chain to 2 other boards by the terminal block. Power should be sufficient. The PSU should operate in its efficient range, and the current on the single wires is not too high.
Your solution indicates a much higher current than the 75W for a PCI cable. Is that OK ?
I don't know if there's a limitation to 75W per PCI-e cable. In my case there would be ~120W per wiring. Do you have a source for the 75W limitation? I'll look into the PSU reference guide.