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