Preheat oven to 375 degrees. Cook pasta according to package directions for 5-7 minutes or until it is a little soft but underdone. Drain and rinse the pasta and set aside.
Process the bread in a food processor until it forms small crumbs. Melt 1 tablespoon butter in a small bowl. Add the breadcrumbs and toss to combine. Add the parsley, garlic powder, Parmesan and 1/2 teaspoon salt to the crumbs and set aside.
In a small bowl combine the milk and cream and set aside. In a medium saucepan melt the remaining 3 tablespoons butter over medium heat. Whisk in the flour and heat until bubbly. Continuing whisking and cook for 1 minute. Slowly add the milk mixture, whisking continuously until smooth. Cook until the milk has thickened enough to coat the back of a spoon.
Remove the saucepan from the heat and add the cayenne, Old Bay, mustard and 1/2 teaspoon salt. Stir in the cheese until melted. Check seasonings and add salt if needed.
In a large bowl combine the pasta, cheese sauce and crab meat together taking care not to break up all the crab lumps. Butter or lightly grease a 1 1/2 quart casserole dish. Pour the macaroni and cheese into the casserole dish and sprinkle the bread crumbs on top. Bake 25-30 minutes or until the top is brown and macaroni is bubbly. If the topping browns too quickly cover loosely with foil.