Imo skip the GPU inlet and have it flowing from ram->gpu->ram.
Gpus dont make that much heat (compared to cpus anyway - and then its a bigger area to cool anyway), even 6800s, itd be nicer with just two barbs.sticking out.
That would ean you need wider channels for the increrased flow, dunno if it would fit..
Also dont use accrylic it cracks too easly. Polycarb is good but there are better plastics that are not transparent, but as the block faces downwards, you wouldnt see much of it anyway.
Of those plastics, ones I can remember are derlin, maybe slightly expensive/hard to find
The other is polyethylene, a good soft easy to machine material that is not brittle so wont crack, and is compleatly fine with water in it. Most plastic containers for solvents/petrol are made from HDPE so its a good choice. Also cutting/chopping boards are often made of it, which is a good cheap source of thick plastic for tops - the same size in accrylic or polycarb would probably cost 5-20x more!
Are you making it by cnc or just a normal mill/other tools?
Anyway make sure you post pics of the finnished block
