Hi atz 75,
Thanks for your comments. My timeline, by the way, is for the movie alone in which the sinking of the Thresher is not mentioned. Indeed, it may be significant that Diana Ossana and Larry McMurtry, in adapting the short story, choose not to include that detail, making way, thereby, for the longer timetable for the summer of '63. But, as I say in footnote 1, the shorter timetable is perfectly workable.
As for the likely weather conditions in Wyoming in the mountains in May - for that, I'd have to check with somebdy else. I have almost all the Call Sheets for the movie and can tell you that the early mountain scenes were shot on 21-23, June. But this, of course, is in Alberta which is considerably North of Wyoming.