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I run #10 from an aeromotive A1000 in tank pump to a Y, where I split to two #8 lines feeding each bank. The return line is #8. Never seen a problem. If I were you, I would go to #8 from the pump and put a regulator as Rick suggested and then use #6 for the return. Do not worry about the #8 line being bigger than your pump fitting ID. Resistance to flow is as much driven by length as diameter. You do not want a long line to be the same ID as some short length fitting.