I have seen the movie in question. I feel it is a load of crap. For one thing, when the James-Younger Gang went into hiatus...Frank and his wife went to Tennessee, Cole and Bob Younger went to Texas, Jim hid out in the hills of Missouri and Kentucky, Jesse's whereabouts are vague but he did keep in touch with Frank. I don't believe however that Jesse went farther East than Kentucky and West Virginia when they resumed their bandit activities. My Great Grandfather( My Grandmother's Dad) born in 1865, grew up in Clay County Missouri. As a boy he knew the James and the Younger brothers. As a matter of fact my family was related to the Fords by marriage and it has been said in our family that the James and Fords were blood related. My G-Grandad died at the age of 98, when I was 10 years old. I remember his stories, told to me, of him, as a lad, knowing and wanting to emulate the James boys. To him they were heroes. After he got into a shooting scrape at the age of 15 in 1880, his mother made him return to Virginia to live with her sister, fearing he would end up like those whom he had admired. According to him he never saw his mother again as she died of typhoid 2 years after he left.
I think you will find that the accuracy of this movie is extremely suspect and the events probably onl;y exist in the mind of a hollywood writer. There may have been a Native American who rode with the Guerillas during the war, but when the James-Younger Gang went on their outlaw raids, it was all family and friends. As it turned out, those friends were not so trustworthy.