So right now you have a '71 K20? And you have a newer '73+ axle, from a K10? If so, they should both measure 9 15/16". I'm assuming on the '71 you're using the snap ring nearest the end of the splines, check on the '73+ shaft you have a snap ring groove in the same spot. As long as both stubs are the same length, and have snap ring grooves in the same spot, run 'em.
