If you already went through the very stressful process of opening and disassembling the unit the CPU replacement shouldn't be a problem. I did it myself though I've used the 6700K - can't see why the 6700 wouldn't work after al my unit had the 6600 (not the unlocked K).
One thing though: the spring that holds the heatsink in place can be a very difficult thing to put back and maybe even damage your CPU - ANY CPU if you're not careful enough.
I recommend ‘flatten’ it a bit using a plier or whatever you can. It should still work as to hold the heatsink in place but if it is too strong that you have to push down very hard (and I am quite a strong person…) you should definitely ‘flatten’ it or you WILL damage you CPU. Check the CPU replacement guide and read the comments on step 62 - you'll see I'm not the only one who had a problem with that:
iMac Intel 27" Retina 5K Display CPU Replacement