Preheat oven to 425 degrees F. Spread the flour on a plate, beat the eggs and milk in a shallow bowl, and spread the bread crumbs on a plate. Season all three with kosher salt. Working a few at a time, dredge the shrimp in the flour, then the eggs and then the bread crumbs. Put the breaded shrimp on a baking sheet. Heat 1 inch of vegetable oil in a straight-sided skillet until the tip of a shrimp sizzles on contact. Fry the shrimp in batches, taking care not to crowd them, until golden on all sides, about 2 to 3 minutes per batch. Drain the shrimp on paper towels, and season with salt. Mix together the mozzarella and grated Grana Padano in a bowl. Spread 1 cup marinara sauce in the bottom of a 10-by-15 inch Pyrex baking dish. Arrange all the shrimp in one layer, and drizzle with 1 1/2 cups sauce. Sprinkle with the cheese mixture, and spoon dollops of the remaining 1/2 cup sauce on top. Place in the oven, and bake until cheese is browned and crusty and sauce is bubbly, about 15 to 20 minutes.