I had a similar problem on my 50 with one rear drum.

I tried a mallet, pry bar.
it ended up being corrosion as seen in this pic
You can see where the black paint is corroded over.
I got it to brake loose by spraying wd-40 in the pac man looking hole on the drum face, and the two round cut outs on the drum

pic 2

Your year truck may not have those holes on the drum

Good Luck



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1950 chevy 3100