Posi rear differential factory option?

[custom10nut]

A few questions here.they come from seeing a SPID for a 72 K-10, (4spd equipped) on a FB truck page.
The SPID listed a POSI Traction Rear differential.
How common are the and/or are they hard to find?
Whatever Gear would have been in this Rearend? ( I’m assuming it would be 4.10)
If I were to find one, would it be possible to change the 4.10 to a more Highway sensible ratio (3.55 or lower )?
Last question would be: what would the Factory Part numbers be?
(So that I know that I’m getting an Actual POSI unit)
My current rear end is a “One Wheeler” and I’d sure like to change it over to a POSI unit.