I believe on a machine in a German lab in the early 70s (I trust that more then than a power meter now) Eddie Merckx did north of 480 for an hour. I got to think being almost 20lbs-40lbs (he was huge as a junior ITT WC) heavier than Eddie Indurain did more on the road, but there are no numbers I know of, just he was big and fast - and could climb.
I expect the PEDs helped a lot for both so maybe they'd be 10%-15% less now?
Those sill are big numbers. Look them up.