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The power booster i pulled from a 78 truck came with a flat plate with rubber seal on one side of it that went on before the bracket, maybe that is what your missing?
I am in the exact same boat as you. My rubber boot is to short. All I can find is a seal for a 65 ford fairlane (part# C0AA-2A021-A). Looks like what we need but I don't know if it will fit. They run $10. Does anyone else know where to get this?
The missing part is a plastic sleeve...as Bill noted above...its notched to go inside of the firewall. Its around 1 1/2-2" diameter..its a hard find on the early trucks....I dunno what the later trucks used for this pourpose.