Yep, I agree with EK&AT - the DD block is made 'simpler', that's why it fits 'better' on the 580.

I just got the 2nd DD block fitted to my 2nd 580, this one took a little more effort to fit - VRMs was not as tight towards the wb as the first one.

In the end, O/C did not gain anything from these 480 waterblocks - it was no difference from the OEM H/S (except the noise, of course):
- 855/2150 w/o voltage hike
- 925/2150 at 1.125v

Good enough for me, but maybe not for everybody else... YMMV.