From what I see it will fit in an oMP without a problem. Supplementary power may be needed, although I have been running a Gigabyte 3 fan overclocked GTX 780 Flashed by MVC in my Mac Pro 5,1 using internal power for 6 months now without incident.
Thanks for the confirmation guys. Got my hands on a oMP and i'm in the process of doing all the nice things you can do to bring it up to speed. Already dropped in a Samsung 840 Pro SSD attached to a SATA III PCIe card and 32GB of RAM. CPUs and GPU are next. Thinking of getting the 780 Classified and a pair of x5660s. Any more suggestions?