Not your normal Mopar cooling issue

Most of the Mopars I've owned that have had cooling issues were the "gets hot idling in traffic" type.
But #projectvertfish has something different.
My BPE 408 runs great and stays cool (180 degrees) at idle and around town but climbs to 195-200 degrees at extended periods at 75 mph or above. 3500 rpm.
I'm thinking it might be lean issue.
Or possibly something to do with using the restrictive cast iron 273 manifolds.
Opinions welcomed.
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