over heating problem

1969 Fiber-form w/225 buick inboard overheating. when idling it cools fine, when a little off idle it cools, but at half throttle it overheats. The lower unit was rebuilt, a new water recirculater was put in, the only other thing that was changed were the heads. I think that they were replaced with automotive grade, could this be the problem. Any information would be helpful. Thank You.

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