I've installed a W3670 in my 5,1 and have used FCPX quite a bit since, works beautifully. Still works in Mavericks as of DP5 as well 😉
Only thing I'd say is if you can find the W3680 or the W3690 for a decent price, that's probably better; the W3670 is great, but it's the last CPU to only run RAM at 800MHz or 1066MHz, and only has a 4.8GT/s QPI link; the W3680 and W3690 run at 6.4GT/s and can run RAM at 1333MHz.
E0 or B1 steppings will work.
Avoid Engineering Samples, they are cheaper but aren't guaranteed to be as stable. I bought a retail W3670 part brand new for about £440, expensive, but the speed difference compared to my W3530 2.8GHz Quad is noticeable.