1. With stiff brush, scrub clams with running cold water to remove any sand. In 8-quart Dutch oven over high heat, heat 1/2 inch water to boiling. Add clams; heat to boiling. Reduce heat to medium; cover and cook just until shells open, about 5 minutes.
2. Discard top shell from each clam; rinse clam on half shell in cooking broth to remove any sand and place on plate. Let broth stand a few minutes until sand settles; spoon 3/4 cup clear broth into 1 cup measure and reserve. Discard remaining broth. Rinse and dry Dutch oven.
3. In same 8-quart Dutch oven over medium heat, in hot oil, cook onion, green pepper, and garlic until tender. Add tomatoes with their liquid, wine, parsley, salt, basil, and reserved clam broth; heat to boiling. Reduce heat to low; cover and simmer 15 minutes.
4. Increase heat to high; add fillet pieces, shrimp, scallops, and crab legs; heat to boiling. Cook 1 minute or until fish flakes easily when tested with fork and shrimp turn pink. Add reserved clams; heat through.