Brass is an alloy that includes copper. There will be galvanic corrosion, but these metals are so close to each other on the galvanic scale, that it's barely worth mentioning. Thermal properties are a different story.
Albiger: go ahead and see what you come up with, and we'll compare notes.
Mag Drive #7
Flow (gph)
700 @ 0 feet
550 @ 2 feet
480 @ 4 feet
400 @ 6 feet
300 @ 8 feet
120 @ 10 feet
max 13'
Here's the data, graphed. Notice the bump in the 300 to 500 gph range: that's your pumps most efficient range. You should target 5 to 8 gpm, which would give you a total headloss of 8 feet and 4 feet, respectively.
About the 3/8 barbs: Ideally, you want to keep the speed of the coolant under 5 fps (feet per second), otherwise the pressure created restricts flow (I'll have to revisit that sometime).
My calculations indicate that a 3/8 opening with a 5 fps (4.9 actually) flow speed will result in a pressure drop of 1 inch of head, for each inch of length of that 3/8 opening, resulting in a flow rate of 1.7 gpm.
Using two of these as outlets, that would give you ideal flow at 3.4 gpm, or ~200 gph.
At that flow rate (3.4 gpm), the 5/8 inlet flow speed is 3.6 fps, with a pressure drop of 1/3 inch, for each inch of length.
If you target 5 to 8 gpm:
5 gpm:
the 5/8 inlet will have a flow speed of 5.2 fps, with a pressure drop of 2/3 inch per inch of length.
the 3/8 outlets will each have a flow speed of 7.3 fps, with a pressure drop of 2 inches per inch of length.
8 gpm:
the 5/8 inlet will have a flow speed of 8.4 fps, with a pressure drop of 1.4 inch per inch of length.
the 3/8 outlets will each have a flow speed of 11.6 fps, with a pressure drop of 4.8 inches per inch of length.
Now of course you want most of the restriction to be in the block, not in the surrounding tubing.
You said that a free flow test got you 6.5 gpm, with both barbs on. That's about 400 gph, which means that you're getting a pressure drop of about 6 feet, according to the pump chart.
At 6.5 gpm:
the 5/8 inlet has a flow speed of 6.8 fps, with a pressure drop of 1.0 inch per inch of length.
the 3/8 outlets each have a flow speed of 9.4 fps, with a pressure drop of 3.3 inches per inch of length.
All this doesn't take into account the rad that you'll be using, if any.
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