Let cooked shells stand in cold water to prevent them from drying out.
Drain spinach well, pressing between paper towels.
Stir together spinach and next 5 ingredients. Spoon mixture into cooked shells.
Spread half of marinara sauce in a lightly greased 13- x 9-inch baking dish. Arrange shells over sauce; top with remaining sauce.
Bake, covered, at 350° for 40 minutes. Uncover, and sprinkle with mozzarella cheese; bake 5 to 10 more minutes. Sprinkle with basil.