Why? Less water circulation? Bad distribution planning? Smaller jacket x-section near the deck? Any chance that it needs a different head gasket (have they changed the gasket since your purchase)? Chevy changed the head-to-block water flow many times between 1937-63 on the L6.
If this turns out to be an issue, I'd consider external water to the rear core plugs like Chrysler did to the Cummins.