You might not need to get the additional gasket. Most A's don't have them installed.
Take it loose from the exhaust manifold and clean the mating surfaces well. Then place a floor jack under the muffler and jack it up so it mates with the manifold. Adjust it so it lines up well. If your muffler flange has a ripple or bend in it you can take a small ball peen hammer and form the muffler flange to the exhaust manifold for a better fit.
While keeping things lined up carefully lower the jack just enough so you can apply a thin bead of muffler cement to the muffler flange. Jack the muffler back into place and install the muffler clamp. Allow the cement to dry for a day before starting.
Having a friend help run the jack while you keep things lined up is a plus.
Also loosening the tail pipe clamp and extra little bit while doing this can help.