Cook pasta according to package directions. Preheat oven to 350 degrees. In a large bowl, combine all other ingredients except marinara sauce. Stir until well blended. In a 13 x 9 x 2 -inch baking pan, spread 1 cup of marinara sauce evenly. Fill each shell with chicken and cheese mixture (about 1 rounded tablespoon), and place open-side up in the baking dish. Pour remaining marinara sauce over the filled shells. Cover the pan with foil and bake for 30 minutes. Let stand for 10 minutes before serving.