I also went from D7000 to D600 and had an L-bracket I needed to salvage. I originally purchased it on the internet but don't know if it is a Kirk or other generic. That said, the mounting slot has ample lateral adjustment and it has been perfectly useable with no issues in my case.
There is a slight raised edge on the bracket that is not perfect fit but still tightens nicely into the base of the camera. The worst that can happen is having to grind the bracket edge down some (which I have not seen a need to do) if you want a perfect fit.