Rex Ingram, playing Jim, was actually born on a riverboat on the Mississippi River. He was born near Cairo, IL, which is Jim's intended destination in the book and film.
When the con-men Walter Connolly and William Frawley advertise "Romeo & Juliet" as the play they were to present, they say it stars "David Garrick" and "Mrs. 'Sarah Kemble Siddons'", two of the most famous British actors of the 18th century. David Garrick and Sarah Kemble Siddons were both long dead by the year in which "The Adventures of Huckleberry Finn" is supposed to take place.
The real-life prototype for Tom Sawyer was Mark Twain himself; the book is largely autobiographical (with some embellishments, of course). Huck was based on a boy Twain befriended in Hannibal, MO, named Tom Blankenship, the son of a local vagrant who Twain said "had the greatest heart of anyone he knew". Becky was based on a girl Twain was seeing for awhile in Hannibal named Laura Hawkins. Injun Joe was based on a local vagrant named Joe Douglas who spent half his time in jail, and who was very sweet to the local kids (when drunk) but could be scary when he sobered up.