Cook the macaroni according to the package directions. Drain well, and set aside.
Melt 1/4 cup butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly until the mixture is thickened and bubbly. Add salt, garlic salt, and pepper, stirring well.
Combine the sauce, reserved macaroni, cheeses, salmon, and eggs in a large bowl; stir well. Spoon the mixture into a greased 11 x 7 x1 1/2 inch baking dish. Combine soft breadcrumbs (just put bread in a food processor to make these) and 2 tablespoons melted butter; Stir well. Sprinkle the breadcrumb mixture evenly over the casserole. Bake at 350 degrees for 1 hour or until set.