Melt margarine in 9x13 pan. Add brown sugar and cinnamon and stir. Lay bread slices on mixture. Beat eggs and salt together in a separate bowl, then add milk; pour over the bread. For best results, cover and place in the refrigerator overnight (or for at least 2 hours). The next morning, uncover and bake at 350 for 45 minutes. Cut into squares and place upside down on plates so margarine and cinnamon are on top.