Our understanding of copyright law is that anything published in America before 1924 is automatically in the public domain, so the only Twain material that's still under copyright would be works like "Letters from the Earth," published long after his death. Stolen White Elephant has, in fact been added to the web by Project Gutenberg at:
http://www.gutenberg.org/etext/3181
Terry Ballard
Quinnipiac University