Imdb has the credits swapped for these two:
Christopher Mitchum as Michael McCandles
Bobby Vinton as Jeff McCandles
I just watched the movie. Christopher Mitchum is clearly the McCandles shot early in the movie and at risk of dying. I cannot remember if he was referred to as Michael or Jeff. However, I did notice that Bobby Vinton is referred to as Michael all through the movie.
Sorry my friend but you have it backwards. Chris M. plays Michael all through the film. If you recall when they stop at creek to rest and water the horses, Jake asks, "Michael, what is that you got wrapped around your waist?", and that was most definitely Christopher Mitchum (watch Chisum and you will see C.M. in that movie too).
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Carpe Carpio
I cannot remember if he was referred to as Michael or Jeff. However, I did notice that Bobby Vinton is referred to as Michael all through the movie.