I'd take what ever he is willing to let you have,i'm assuming you mean its a Pre-Engineered building company like for example a Butler or American Building.
It also really depends on what kind of tools and equipment you have at home to work with to cut up and fabricate a bumper with..welder..cuttin torch,,grinder,cut wheels...etc etc.
Most of the Girders and Perlins i've ever seen on Pre Fab buildings are 1/8" to 3/16" material..which is pretty common for a bumper,plus the knee brace material,bridging stiffeners for the perlins...etc
Anything from sheetmetal up to 1/4" material would be good scrap to have around for jeep projects
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