Rear bumper bracket set with hardware for my 1968 GMC K20 long bed fleet side frame

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I’ve been having trouble finding the rear bumper bracket set with hardware for my 1968 GMC K20 long bed fleet side frame. I’ve looked at the main online sites, googled like mad, and called local retailers. I can find ones for the short bed, but was told by Classic Industries that these won't work. Has anyone had luck finding these parts? Thank you.