1. Preheat the oven to 400 degrees F. Grease a 12-inch cast-iron skillet.
2. Fill a medium saucepan three-quarters full with water. Bring to a boil over medium-high heat, and add the shells. Cook the pasta until almost al dente, 5 minutes. Drain the pasta in a colander placed over a bowl. Reserve 1⁄4 cup pasta cooking liquid, and discard the rest. Pour the pasta shells into the skillet.
3. In a medium saucepan melt 2 tablespoons of the butter over medium heat. Add the garlic and flour. Cook for 1 minute. Gradually whisk in 1 cup of the milk until smooth. Add 1 more cup of the milk. Stir and cook for 3 minutes. Whisk in the cream cheese and2 cups of the Cheddar cheese until smooth. Stir in the salt, sugar, cayenne, reserved pasta cooking liquid, pimientos, and the remaining 1⁄2 cup milk. Cook for about 5 minutes, stirring occasionally. Remove the saucepan from the heat. Pour the cheese sauce over the pasta.
4. Sprinkle the remaining 1⁄4 cup cheese on top, and then evenly cover with the bread crumbs. Cut the remaining 1 tablespoon butter into small pieces, and scatter over the bread crumbs.
5. Bake for 20 minutes, or until golden brown. If it has not reached a golden-brown color, broil on high until golden brown. Let stand for 5 minutes before serving.