A restrictive rad doesn't really help this setup. The point of the setup is to have a VERY free flowing rad, so you have more water going though it than the blocks do. The valve and recirculation line allows you to have more water passing though the radiator.
I think the pumps in paralell would help the flow, IF you went with 1/2 tubing from each pump, then joined them in a single 3/4 line, if you used 2 half inch lines into 1 half inch line, you would be choking off the flow.
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