Exhaust Manifold to pipe problems

So I've had this 71 Demon with the /6 for more than a year now, a few months ago we replaced the exhaust manifold (had a huge crack in it) ever since then the gasket between the bottom of the manifold and the exhaust pipe keeps busting. I've been replacing it with a flat gasket but the other day a guy a the auto parts store said it shouldn't look like that, something about a round metal piece or two? any ideas or pictures? This is really starting to kill my millage.