Preheat oven to 425ºF (220ºC).
In large Dutch oven, heat vegetable oil over medium heat; cook celery, onion, green pepper and basil, stirring occasionally, until vegetables are softened, about 8 minutes.
Add tomato paste; cook for 5 minutes, stirring occasionally. Add plum and crushed tomatoes, salt and pepper; bring to boil. Reduce heat, cover and simmer for 2 hours.
Uncover and simmer for 1 hour or until sauce holds ripple when pushed. (Make-ahead: Let cool for 30 minutes; refrigerate until cold. Refrigerate in airtight container for up to 3 days or freeze for up to 2 weeks.)
In large pot of boiling salted water, cook pasta shells until tender but firm, about
8 minutes. Drain and return pasta to pot. Add 2 cups (500 mL) of the tomato sauce; toss to coat.
Ladle 1 1/2 cups (375 mL) more sauce into 13–x-9-inch (3 L) glass baking dish; spread half of the pasta over top. Repeat layers once; top with 1 cup (250 mL) more sauce. (Make-ahead: Cover and refrigerate for up to 1 day; uncover and heat in 350ºF (180ºC) oven until heated through, about 40 minutes.) Reserve remaining sauce for another use.
Overlapping slightly, arrange Cheddar and mozzarella cheese evenly over top; bake in oven until cheese is melted, about 5 minutes.