I suspect there may be air trapped in the back of the engine in the coolant circuit, I ran into this when I first put my 455 together. I know at least on my newer engine I ran coolant out the back of each head to the water crossover and no problems to date. I know there used to be aftermarket manifolds that had 2 holes on the crossover and depending on early or late heads 55-64 would require soft plugs to plug up one set of holes, but since this is a custom crossover that shouldn't be an issue.
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Illinois Outlaw Gassers
6.27@107
9.97@131
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