Awesome cube mod, basically you halfway built a new one from scratch!
A 760 ti should be possible to get to run inside the 200W power envelope. I have a AMD HD7850 running off a Pico160xt and Dell DA-2, and thats rock stable. My CPU should draw a little bit more than this one (Xeon E3 1230v2) too.
I'd go for undervolting at stock speeds in any case. But i am unsure if this can be done vis the card BIOS for nVidias, or if there is a way to do it in software under OSX.