Prepare pasta shells in boiling salted water as label directs. Drain shells and rinse with cold running water to stop cooking; drain again. Arrange shells in single layer on waxed paper and set aside.
Meanwhile, in 4-quart saucepan, heat olive oil over medium heat until hot. Add onion and cook until tender, about 5 minutes. Add garlic and cook 1 minute longer, stirring frequently. Add clam juice and cook 7 minutes over high heat until reduced to 1/2 cup. Stir in tomatoes with their puree, breaking up tomatoes with side of spoon. Add tomato paste, sugar, and crushed red pepper; heat to boiling. Reduce heat to low; partially cover and simmer about 20 minutes, stirring occasionally. Stir in cream and cook 2 minutes longer; remove saucepan from heat.
Transfer 1 cup tomato sauce to 3-quart saucepan. Add shrimp and scrod, and cook over medium-high heat until seafood just turns opaque throughout, about 5 minutes, gently stirring occasionally. Remove saucepan from heat; stir equal amounts of frozen peas into both saucepans.
Preheat oven to 400°F
Fill each pasta shell with 2 heaping tablespoons seafood mixture and place in 13x9-inch glass baking dish. Pour tomato sauce over stuffed shells.
Prepare Bread-Crumb Topping:
In nonstick 10-inch skillet, heat olive oil and garlic over medium heat. Add bread and cook until golden, about 5 minutes, stirring often. Discard garlic.
Spoon bread crumbs over stuffed shells. Bake 20 minutes or until hot and bubbly.