Preheat the oven to 350°. Butter a 7"x11" casserole dish.
In a skillet over medium heat melt 4 T. of butter. Add the shrimp and cook just until shrimp is opaque. Using a slotted spoon or a spider, remove the shrimp and set aside. Whisk in the flour, and, whisking constantly, cook for a minute or two. Gradually add the milk, whisking until thick. Take off the heat and stir in ½ cup of parmesan and the salt and pepper. Set aside.
In a medium mixing bowl mix together the ricotta, mozzarella, egg, nutmeg, garlic powder, and parsley flakes. Fold in the shrimp and crabmeat.
Spread 1 cup of the sauce in the casserole dish. Fill shells with 2 T. of the cheese and seafood mixture, and arrange them in a single layer on top of the sauce. Spoon the remaining sauce over each stuffed shell. Sprinle the remaining parmesan over everything.
Melt the remaining 2 T. of butter. Toss the breadcrumbs with the melted butter, and sprinkle over the shells. Bake for 30 minutes or until bubbly and golden brown in spots.