Seeing that you're already down to 6.5 gpm with your Alu prototype, 5 gpm would be a target. A definite minimum.
I found an absolutely amazing link:
http://www.lmnoeng.com/index.shtml
I'll be using it to help calculate the resulting flow restriction through your block (and mine!).
As for the flow restriction through the rad, there's no way to tell, because there are no graphs available to give us such info. However...
We might find a chart for a similar core
over here .
Looking at big momma, I'd extrapolate a pressure drop of 5 psi@5 gpm. Dang, that's 11.5 feet of head!