Cook shells according to package directions; drain.
Meanwhile, melt 3 tablespoons butter in a large skillet; add mushrooms and garlic, and sauté until mushroom liquid is absorbed. Transfer mushroom mixture to a large bowl.
Melt remaining 1 tablespoon butter in skillet. Add spinach; cover and cook over medium-low heat 5 minutes or until spinach wilts. Add spinach to mushroom mixture. Stir in ricotta cheese and next 4 ingredients. Spoon spinach mixture evenly into shells. Place stuffed shells in a greased 13" x 9" baking dish.
Melt 1/3 cup butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in half-and half; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in 1/2 teaspoon salt. Add Gouda cheese, stirring until cheese melts. Pour over stuffed shells.
Process bread in a blender or food processor until it resembles coarse crumbs. Spread crumbs in a small pan; bake at 350° for 3 to 4 minutes or until toasted. Combine toasted crumbs, cheese blend, and 3 tablespoons melted butter in a bowl; toss well, and sprinkle over shells.
Bake, uncovered, at 350° for 45 minutes or until bubbly.
Note: If desired, cover and chill manicotti at least 8 hours before baking. Remove from refrigerator, and let stand at room temperature 30 minutes. Bake as directed.