Pat roast dry and rub all over with 1 tsp. salt; set aside. Puree anchovies, garlic, rosemary, orange zest, oil, pepper, and remaining 4 tsp. salt in a food processor until finely chopped, about 1 minute. Transfer all but 1 Tbsp. garlic mixture to a medium bowl; reserve remaining garlic mixture in food processor. Rub roast all over with half of garlic mixture from bowl. Place fat side up on a wire rack set in a rimmed baking sheet or in a roasting pan fitted with a rack. Add breadcrumbs to remaining garlic mixture in bowl, mix thoroughly with your fingers, and press onto top and sides of roast. Let stand at room temperature at least 1 hour. Meanwhile, add butter and parsley to reserved 1 Tbsp. garlic mixture in food processor and pulse until well combined. Transfer to parchment paper and roll into a log. Chill until firm, about 2 hours. Place rack in middle of oven; preheat to 500°F. Place roast in oven, then reduce temperature to 350°F. Roast until an instant-read thermometer inserted into the center of roast registers 120°F for rare or 130°F for medium-rare, 7090 minutes. Transfer roast to a carving board and let stand 15 minutes. Slice against the grain into 1/3″ pieces. Serve with herbed butter.