Sometimes it is possible to unbolt the backing plate and after removing wheel bearing lock nuts pull the entire wheel end assembly; hub, drum, and all brake parts.
Then reaching in the hole in backing plate you can fiddle with brakes enough to free the drum.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-