I am thinking of going big on the PSU. Like 900w big. Overkill?
Very. Clicking on the PC Parts Picker link for the build above says the estimated power draw is 598w. Erring on the safe side and adjusting for OCing might give you 800w as an upper number, but a solid 750w unit would be more than enough (any Gold/Plat rated unit by Corsair/Seasonic/Rosewill).
I ordered everything in your single 7970 build minus the optical and case. Still looking at cases. Just ordered the Seasonic 850 and the ASUS GPU. So 90% done.