Boil shells in water with the tablespoon salt until fully tender. Drain, rinse briefly under cool water and drain thoroughly.
Toss pasta with 2 tablespoons pickle brine and let stand for 5 minutes. Mix remaining brine with mayonnaise, sour cream, Dijon, garlic powder and pepper.
Fold creamy dressing through the shells with drained pickles, cheddar, red onion and dill.
Cover and refrigerate for at least 30 minutes so the pasta chills and the flavors settle; the listed prep time includes this chilling.
Stir before serving and keep refrigerated until needed; refrigerate leftovers within 2 hours.