The 71 high gears I have are a few 1000 miles from where I am but check the parts books and you can see the number teeth on the matching high gear and fixed layshaft gear changed, even if it was only on one gear then they both cannot fit earlier years as they changed both teeth profiles, the gear that dogs into it also changed but not in gear teeth just the way it mates with the fixed layshaft gear. This arrangement was a very short lived one as the 5 speed replaced it.
The high gear goes from 26T which it had been for years to 25T, from memory the layshaft went from 20 to 19. The effect was to thicken up the gear teeth but why they did this only a year before the 4 speed died you have to wonder, no doubt 100% 5 speed was not expected. Trouble is in 69 they changed a lot of dimenions buy 2 or 3 thou so mixing 70 box parts with earlier ones is fraught with issues, so a complete box may be the best answer.