A pic of my spid on my 72 K10 from my build thread.
https://67-72chevytrucks.com/vboard/...2&postcount=14 Originally factory posi and 3.73 gears. If the gear ratio is not listed on spid it would be standard. Not sure what the standard gear is on a 72 4x4, would have to do some research. Believe the cutoff on the carrier is 3.73. If you want higher gears than that a new carrier is required or a carrier with higher gears than 3.73. Probably easier to get a new posi unit of your choice to go with new gears. Unless you are looking to only have original OEM parts in the truck there is no reason to install a 50 year old used posi unit. The clutches most likely will have significant wear. Who knows? You may get lucky. Just in cast you are not aware the carrier is the differential or Posi.