The rear drum on a GMC rear-end is not the same as the front drum. The GMCs had rear leaf springs, not coil springs, and as a result, the rear drums are fatter from outside to inside.
I ended up getting some Wagoneer drums that were the right pattern but wrong hole sizes, and had them machined. It was expensive, but I am confident the shoe is making full contact with the drum, unlike going with a narrower drum. You can read the specifics in
this thread. (as previously posted by MusicMan70)