Bake potatoes at 375° for 1 hour or until tender. Cool. Cut potatoes in half lengthwise. Scoop out the pulp and place in a bowl; set shells aside. Add butter to pulp and mash. Beat in milk, mayonnaise, salt and pepper until smooth. Fold in crab. Spoon into potato shells. Place in an ungreased 11x7-in. baking dish. Sprinkle with cheese if desired. Bake, uncovered, at 350° for 30 minutes or until heated through.