Maybe someone can verify this ...
I've heard anecdotally that because the HO72 axle (for our 1-tons) was made all the way through the late '60's, you might be able to retro fit the newer Bendix style backing plates, shoes, mechanisms and drums from a newer HO72 to an older one.
If you can find a '68 3/4-ton truck with an automatic transmission and coil springs in a junkyard, there is *possibility* that you can find the 4.10 gearset as well as better brake components ... i.e., grab the whole axle.
But this needs verification.