63 dart exhaust manifold

Well I put my new intake/exhaust gaskets on , and took for a shake down cruise, when I came back, seen the exhaust manifold had cracked. I torqued to specs(10 lbs), don't know what happened. Anyway I'm looking to buy an aftermarket manifold. I see A1 auto has one for around a hundred bucks, but the Dorman is around three hundred,any difference. Will JB weld work on the crack. Thanks Jim.