Preheat the oven to 350°F and spray a 9x13-inch baking dish with cooking spray.
Cook the jumbo pasta shells in a large pot of salted boiling water according to package directions until al dente; drain and set aside to cool slightly.
In a large bowl, combine the ricotta, egg, 1 teaspoon Old Bay, 1 teaspoon Italian seasoning, Dijon mustard, 1/2 teaspoon salt, and 1/2 teaspoon pepper until smooth.
Fold the cooked shrimp and diced or shredded imitation crab into the ricotta mixture until evenly distributed.
In a separate bowl, whisk together the Alfredo sauce, heavy cream, 1/2 cup Parmesan, 2 teaspoons Italian seasoning, and salt and pepper to taste.
Pour about 1/2 cup of the sauce into the bottom of the prepared baking dish and spread to cover.
Fill each cooled shell with about 2 tablespoons of the seafood-ricotta filling and place the shells open-side up in the sauce-lined baking dish.
Pour the remaining sauce evenly over the stuffed shells.
In a small bowl, mix the bread crumbs, 1/2 cup Parmesan, melted butter, and 1 teaspoon Old Bay until combined, then sprinkle this crumb topping over the shells.
Bake for 20–25 minutes, until the shells are heated through and the topping is golden brown.
Remove from the oven and let rest for a few minutes before serving.
