I've had the same situation in inline sixes over the years.
One thing I've done to combat it is to open up the block-to-head transfer holes at the rear, and close off the ones near the front. This forces more of the water all the way through the system rather than allowing some of it to return via the front (something Buick apparently had already done a little of when they re-engineered the head gasket). Had to be careful to avoid restricting the total flow.
I'm looking at this inline eight, and considering a mid-point entry as well, as you've done. Another possibility may be an internal "dispersal" tube like Chrysler used on their inline flatheads.
I'm also giving some thought to either mid-point (a la Ford's flathead eight) or double end collection from the head.
Have to go down and look over the blocks & heads again this evening, with tape in hand, and your cobble in mind as well.
Last edited by Old6rodder; 03-01-2018 at 12:02 PM.
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