Wouldn't it be better to just have the water flow from one end of the block to the other, instead of over the center and out both sides? (I'm assuming that's what you're doing due to your "more pressure" comment). That way you can simplify your tubing layout and still get the same performance thereabouts. I'm not sure about the performance hit (in terms of temperature) from having the coolest water be warmed up slightly by the block as it travels from one side, to the middle, then to the other side. My feeling is it wouldn't matter, since the water in your whole setup is going to heat up anyways, and the temperature delta for water coming into the block is going to be marginal compared to water exiting the block; the radiator is cooling everything anyways and it's all about how fast that thing can dissipate heat. Then again I've never made a block before so I don't know.