Well, just for you, I ran out to the garage and popped (partially) the section off the 80's box that contains the input shaft and seal. Look like you'll have to do a basic break down of the box, R&R most of the guts, then press the input shaft and bearing out of the housing to get to that seal. If anyone knows different, please chime in! The 62 and earlier box is much easier, just spin the large bearing preload cup off the top, R&R the seal, and back on.
You should be able to get just that seal. If you go for the break down, make sure you keep things extra clean.