I would like to clarify where the 67 to 72 figures came from. There has been in the past, new gear sets available in 3.90 and 4.10 that were being made for these rearends. The only catch is that they only work in the pumpkins from 67 to 72 because of some internal differences. As an assembly however, all the front loading 3/4 and 1 Ton pumpkins from late 47 to 72 will bolt into the AD trucks. That is my understanding. So one of the routes that could be taken was to buy a 4.57 from a 67 to 72, install the new gears and that would work. I am not sure that the new 4.10 gears are not still available.
