I'm sure the Hatfield/McCoy thing dates back to the 1960's when hillbillies, Hatfield/McCoy, were popular in culture. Remember, prior to a strict adherence to Ozarks culture, the park was a "frontier town" with a sign on the general store that stated they had "supplies for wagon trains heading west," and early special guests to the park as added attractions were from TV westerns of the day. I think Gabby Hayes was featured at one time, I think cast members from "Wagon Train" TV show were there, too.
--
In regards to the Huck/Tom thing, it previously was stated by another that they are part of Missouri history/folklore/literature and more than likely shooting for a general time period thing there that is still recognizable by today's audience.