[justcuz]

See if that bar cross references to an 80's Ford F-250 with the semi-float Dana 60 rear axle. You might get lucky and find that the backing plates and brakes could be the same. Ford used the semi-float Dana 60 from 1980 untill they started using the Sterling rear axle (around 85-86).
In the 60's some F-100's had a Dana 60 semi-float rear axle, probably just as "unique" as your GMC rear end, but may be another source for cross referencing parts.
I don't know what size the brakes are but I am guessing they are smaller than the regular brakes on a full floater Dana 60, otherwise you could just use those parts. I know the hubs and drums would be different but you may be surprised at how much of the stuff from the baking plate out may interchange.