I'd have the parallel Deeps if I had the flow rate/speed.
5/8 or 3/4 tube & fittings also. I'd run it all in series, you loose half the flowrate whenever you go parrallel,. I'm sure the Iwaki could handle a GPU & CPU block in series, even a NB one too. I'd think about having the GPU & NB blocks custom made(in~out, no maze/channel stuff!), with soldered on 1/2~5/8inch internal Copper tube instead of threaded barbs(barbs take up too much room on small blocks. Custom top for the Atlantis as well with 1/2 or 5/8 inner Copper tubes as well(they are very close fitting!!). I think Cathars block will take a 5/8 barb as well,(it has a big threadsize 1/2inch barb on it as standard...
Rotor does one~off custom jobs...
Pressure/head over flow is my take aswell, as long as the flow is respectable, the 20RZT seems OK to me @ 3.5+GPH @ 8FT...
I'm no expert though, I have little or no practical experience

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I wish you luck on this cause it's very close to what I want to end up with

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