I like to use factory or factory type gaskets because they are quite thick. I think this helps with sealing but also with heat isolation, as in keeping the engine heat away from the manifolds and carbs better. Some aftermarket manifolds come with new gaskets but they're very thin paper. I won't use those. I apply a small bead of sealer to one side of the gasket, the manifold side. This is not so much for sealing but rather so that the gasket comes off with the manifold any time I remove it, and doesn't stay stuck on the engine. Since the gasket is so thick, several re-torques of the mounting screws is usually required before they hold tight (due to gasket compression).