Rob, these are the nos stamped on the pinion.
560140-21-48-GM 81-7
On the casting on the crown wheel it is 558912 GM50.
21-48 is the tooth count Mike, that makes it a 2.29 ratio. The carrier is an open 2 spider & probably a 26 spline. If you're going to run that, check the spider gear cross pin "locating bolt" to see if it's broken.
Don't run a collapsible pinion spacer, turn up a solid one that's about .020" wider than the old one & trim it down until you have bearing preload.
Keep the same shim pack when you change the bearing to retain pinion depth.
Sid.