8 tablespoons (1 stick) unsalted butter, plus more for casserole
6 slices white bread, crusts removed, torn into 1/4- to l/2-inch pieces
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons coarse salt, plus more for water
1/4 teaspoon ground nutmeg
1/4 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper
4 1/2 cups (about 18 ounces) grated sharp white cheddar cheese
2 cups (about 8 ounces) grated Gruyère or 1 1/4 cups (about 5 ounces) grated Pecorino Romano cheese
1 pound elbow macaroni