"I'm pretty sure it was him that hit one completely out of RFK, but the astonished umpires called it foul, though many others thought it to be a fair ball."
Yes, he did. Line drive over the left field roofing. So high that the umpire couldn't judge the foul-pole properly. The opposing left-fielder said, after the game of course, that, yes, it had been a fair ball.
Howard also hit one into a stairway in the upper deck.
He also hit one of the most powerful singles of all time: line-drive over short that ripped the short-stop's glove off and carried it into left-field.
I saw Killebrew and Allison and Lemon and Sievers, and I've seen the current players who always swing for the home-run, but nobody hit the ball as hard as Frank Howard.