Place the cooked pasta in a large mixing bowl. Set aside. Preheat the oven to 375 degrees.In a medium sauce pan, melt the butter over medium to medium high heat. Once the butter has melted, add the flour and whisk to combine and let the flour taste cook out, about 1-2 minutes. Add the milk and continue to whisk for about 2 minutes or until the mixture is smooth and slightly thickened. Turn the heat down to low and add the cheeses in, about 1/2 cup at a time, whisking to combine. Keep adding the cheeses in batches and whisking until all incorporated. Finally, add in the Old Bay seasoning and salt and pepper, to taste. Take off the heat.Pour the cheese sauce over the cavatappi. Mix well to coat all of the noodles. Add the chopped bacon and crab meat and gently fold into the pasta and cheese sauce. Place the mixture in individual ramekins or in one 9x9 baking dish.Mix the bread crumbs and butter in a small dish and sprinkle on top of the macaroni and cheese. Bake for about 30 minutes or until the tops are golden brown. Top with lots of fresh scallions and serve immediately.