Do they still use non-regulation baseballs for the HR derby? They used to use smaller ones, they fly out better. IF so, you can't really compare to real home runs for distance.
Not only that, but the sign most of the home runs other than Hamilton's in the derby wouldn't have been home runs in the configuration when Mantle played.
Dimensions during Mantle's playing days (LF -> RF)
1951 - 1953: LF line 281 (but just a few feet to the right, 395), 456, 490, 429, 295
1954 - 1968: 301 (quickly to 402), 457, 461, 407, 296 (quickly to 344)
The HR that was still going up when it hit near the top of the facade was 374' horizontally from the plate, 108' in the air, and bounced upward (as the ball was still going up) and back to the infield dirt, nearly hitting second baseman Jerry Lumpe on the fly. Although home plate today is 30' to the facade (now lights) than is was then, balls that landed well short of where Mantle's blast would have landed were given distances of over 500'. In reality, if a ball were hit 500' to right-center, where the signs are, it would have left the park and crossed the tracks. In dead center, that wall is 505' from the plate