1. Prepare pasta shells according to package directions.
2. Meanwhile, drain chopped spinach well, pressing between paper towels.
3. Stir together spinach, chicken, basil, and next 4 ingredients. Spoon mixture evenly into shells.
4. Spread half of jarred Alfredo sauce in a lightly greased 13- x 9-inch baking dish. Arrange stuffed pasta shells over sauce, and pour remaining sauce over shells.
5. Bake, covered, at 350° for 40 to 45 minutes or until filling is hot and sauce is bubbly. Remove from oven, and let stand 10 minutes.
Note: To make ahead, prepare recipe as directed through Step 4. Cover and freeze up to 1 month. Thaw in refrigerator 24 hours. Let stand at room temperature 30 minutes. Bake, covered, for 1 hour and 20 minutes.