I don't know if this has been mentioned, but I stumbled on to this:
Walt was not only fascinated by the old west, he enjoyed those who explored its myth. He named his riverboat after one of America's greatest storytellers, Mark Twain. Like many things in the park the boat was built in 5/8 scale. The park claimed that this was the first paddle-wheel boat built in the U.S. in over 50 years. The ship was constructed at the Todd Shipyards in San Diego. Disney lore said that it was shipped deck-by-deck up the highway and then assembled at a Disney soundstage. A New York Times article from 1955 read, "Its piece de resistance is another river, plied by the 'Mark Twain,' a self-propelled reproduction of a Mississippi stern-wheeler."