You might be able to pull off a 360 rad since the max power of the 670 is 195W. 400W can be easily managed with the right fan/rad combo.
There are some bayres/pump housings that take up 2 drive bays, and will cost much less than that Koolance unit. You'll only need a fairly small pump since the restriction in a small loop like that is very small; something like a Swiftech MCP350 or MCP355.
The GTX 670s are 0.5" shorter than the 680s, not 4" But either way the blocks should still line up appropriately. I'd consider the Swiftech MCW82 universal blocks + the SLI linkage they make if you want to cut down on costs. Otherwise you'll have a few options with full cover blocks.
EK are doing a bridge edition of the Supreme that maybe worth checking out too, avoid the nickel ones of course, but you wouldn't need a special connector, just some right angle barbs/compressions, although it says designed for the connector block, you don't need to use one http://specialtech.co.uk/spshop/customer/EK-VGA-Supreme...
I'd also look for more rad there, a single 360 isn't much to cool a clocked Cpu and two cards, throw another 240 in and I'll okay it though
**Edit, or design an external radbox, its the new thing in W/c dont you know
Oh, and when I spoke of the "short PCB", this is what i was refering to:
While the card IS only a hair shorter then the 680, when the cooler comes off for the waterblock, the remaining cirtuit board is only like 7 inches long! Im thinking that this is a "good thing" for stuff like case air circulation. But somehow I dont see standard 680 blocks being compatible here... the components are all in different places.
Dinky hehe, sure its not an Agp card?
I would consider putting the Cpu on the proper loop as well, the H100 is taking up vaulable room in there you could use otherwise,
if you do use universal blocks, don't forget your Vram heatsinks, mosfets are covered on that one so you're ok there