Yes, I probably will break quite a few endmills
I'll try to be careful though.
And I'm not worried about having to mill down the copper to size, its just that anything over 1/2" thick seems to be really expensive to purchase (like, twice the cost or more). That's why I would consider making my block a 3 - layer design where all 3 pieces could be milled out of copper that is 1/2" thick. As my design stands now I would need one peice at least .625" and one .875" thick. I can find these thicknesses, and larger, but they are not too cheap.
I will be milling the block myself so I don't forsee too much manufacturing costs (other than lots of my time).
About your suggestion - is it to leave all the pins as high as the center ones, and leave the sidewalls that high, or just to leave the sidewalls that high? Either way - I want a very contolled flow through the block with not much water contained in the block. Thus if I left the sidewalls high there would be a much larger volume for water to take up and therefore a much slower water velocity in some parts of the block. Sure I could leave a protrusion sticking down out of the cover to counteract this - but that seems to be about the same amount of milling as I have now.
Anyway - still working on this redesign, I will post more CAD pics later tonight or tomorrow maybe....