5200 was rated horsepower peak for a 383 magnum, not the redline.
440 magnum hp was rated at 4600. Do you think it quit at 4700? Original Sixpack motors rated at 4700 were failing when owners spun them to 6000 in 69, so Mopar beefed them up in 70.
(and i dont for a minute believe a stock 74 360 Duster ran anywhere near a 13.50. 15.50, I would absolutely believe. Those "pure stockers" are NOT stock!)