76 Duster....Manifolds to Dual Downpipes

You probably don't need a gasket or seal ring at the headpipe-manifold junction. None was used from the factory except in the late '70s-early '80s with the self-contained heat riser valve. Those used a ring seal between the heat riser assembly and the exhaust manifold outlet, but none between the heat riser assembly and the headpipe. If you turn out to need a seal ring, p/n 5013 446AA from the dealer ought to do the job for you.