The Road Runner 383 for that year is listed as 335 brake hp at 5200 rpm. My source is The Standard Catalog of American Cars: 1946-1975, page 675.
Keep in mind that this was in the height of the "horsepower wars" in which car makers were testing engines without necessay accessories to get higher horsepower readings, and insurance companies were in turn raising the coverage rates, which forced car companies to publish lower horsepower figures, etc, etc.
So...you can't really trust any of the published figures for this era, but it is pretty much what everyone goes by anyway.
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