[SS Tim]

Since HO52 were the standard axle in K20 trucks I doubt it is a HO72 as it would have had to have been swapped. Did you actually measure the drums internal size or is this a guess? The outside diameter of the 11" brake drums is right at 13" (see picture). Did you check the axle code stamped into the carrier on top (see picture)?

The No-Spin locker is interchangable between either axle as are most parts including the drop out as a whole and is the only drop-in locker/limited slip I have been able to find for the HO52/72 axles.

A later 14-bolt Detroit Locker can be fitted to the earler HO52/72 differential case assembleis but modified 14-bolt axle shafts must also be used due to side gear spline count differences.

On the brakes a couple of things to note.
11x2-3/4 drums are availabe from Centric with two different p/n depending on year due to stud size changes.
13x2-1/2 drums are very hard to find at all in usable condition. If you find a souce for them new please let me know.

While a 14-bolt intended disc brake kit can be used/adapted to the HO52/72 axles the problem of a parking brake comes up. Some kits do not have one and others use mechanical calipers intended for a much lighter car. While this might be acceptable for some it is not for a loaded truck the weight of a K20 and even less so if it has larger than stock tires.