JR's headstone states that he was born in 1939 which would have made him 74 at the time of his death. According to the TV movie, Dallas the Early Years, JR was 15 in 1951, which would have meant he would have been born in 1936 and he would have died at age 77. Considering that he looked rather aged in the reboot and there was no reason to make the character younger after he was already dead, staying consistent and giving JR's birth year as 1936 made far more sense.
J.R.'s letter begins, "John Ross if your reading this..." It should read "you're."