Ok I've done my shopping and have a length of 3/4'' hard drawn copper pipe (that I'll flare and use as hosebarbs) it has an internal area of 227mm/2, after attacking the heatsink with a ruler and some educated guesswork on the thickness of the fins I've come up with a "channel" area of 432mm/2.
That's actually ideal for the 1'' I was originally planning, but I don't think I'll have room for the geometry needed to get that flow too all the fins properly, so 3/4'' it is. However since I don't want to have the water flow slowing down in the block (re; water block theory) I'll direct the water flow through only the centre of the block, through a roughly equivilant fin area. like so;
this also has the benifit of greatly simplifing the construction, something I won't be complaining about
