Mag3 is a pretty beefy pump; I think you'll be fine. Is your modded 1 pass core now set up to have 2 barbs on top and one on bottom then? If so you could try spliting the flow at the whitewater (use the 2 outlets to go 1 to nb and 1 to GPU) and then run them both to the chevette's dual inlet barbs. You'll lose a bit of cooling performance on GPU and NB but should increase flow rates on the CPU block a bit.
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