Arrange cake cubes in a single layer on a jelly roll pan. Bake at 350° for 15 minutes or until toasted, turning once.
Place 1/2 cup sugar, sour cream, and cream cheese in a large bowl; beat with a mixer at medium speed until well combined. Gradually add milk, beating until smooth. Add toasted cake cubes; fold gently to combine.
Place remaining 1/4 cup sugar, vanilla, salt, and sweet potatoes in a large bowl, and beat with a mixer at medium speed until smooth.
Spoon half of cake mixture into a trifle dish or 3-quart glass bowl; top with half of sweet potato mixture. Sprinkle 1 tablespoon coconut over sweet potato mixture; top with half of whipped topping. Repeat layers; sprinkle with remaining 1 tablespoon coconut and pecans. Cover and chill at least 1 hour.