Also, the gil riser/manifold flange does not flow water directly through it. water comes out of the manifold at the top and routes to the riser through hoses.

interesting.

I copied the way the merc manifolds and risers were plubed up...fresh water in block and manifolds, raw out of heat exchanger and into the risers to be dumped overboard. Looks like it should work fine. These are stock 330 HP engines so I dont see a big problem.