What are you using to overclock it? There are a couple programs that let you unlock the overclocking range & voltage range. You should use the search function to find the 6950 unlocking thread. Hundreds if not thousands of people all over the internet have posts about this.
As far as water block, it depends on the layout. You need to research 6950/6970 water blocks and check compatibility.
if you build it, they will hate