Old men remembering his time as young boy living with Native American

At the end of this movie, an old guy see a letter or a diary and is remembering about the time he spend in an Native American tribe (being a young boy), where he became friend with an native american boy roughly his age (~ 10 y.o.) and also fell in love for a girl in this tribe. It probably was an movie from the 80’s, with small chance to be in the 70’s of 90’s

“Where the River Runs Black” 1986?

The movie you’re looking for is “Little Big Man” 1970