Preheat oven to 400 degrees. Bring a large pot of water to a boil. Butter a shallow 4-quart baking dish (oval or 9-by-13-inch rectangle); set aside.
Generously salt boiling water; add pasta. Cook, according to package instructions, until 2 minutes short of al dente. Drain pasta, and return to pot.
While pasta is cooking, melt butter in a large saucepan over medium. Transfer 2 tablespoons melted butter to a small bowl; set aside for topping. Add flour and mustard powder to remaining butter in saucepan. Cook, whisking, 1 minute (do not let flour darken). Whisk in milk. Bring to a boil; reduce heat to medium-low, and simmer until sauce is thickened, 2 to 3 minutes.
Remove sauce from heat. Gradually whisk in cheeses; add Worcestershire sauce, and season generously with salt and pepper. Add sauce to pasta, and toss to combine; transfer to prepared baking dish.
Place bread in food processor; pulse until very coarse crumbs form (you should have about 3 cups). Add reserved melted butter, and pulse just to moisten. Scatter crumbs over pasta in baking dish. (If making ahead, cover and refrigerate up to 1 day.)
Place dish on a rimmed baking sheet, and bake until topping is golden and sauce is bubbling, 15 to 20 minutes. (If refrigerated, bake for 30 to 35 minutes.) Cool 5 minutes before serving.