Here's another view, with an ATX sized power supply under the PCI slots.
An ATX power supply would fit, but you'd lose the bottom PCI slot.
There's not enough room for both the ATX supply and both PCI slots, not unless I increase the height another 1" or so.
And it's already at 5.25" in height, so I'm trying not to make it any larger.
Do you really want a ATX PSU, presumably a AC-DC one?
If so, I might be able to make an alternate configuration for you when I send the design out for manufacturing.. I'm not expecting it to be cheap, though.