Preheat oven to 350 F. Place bread cubes in 8-inch square baking dish. Sprinkle grapes over bread cubes and toss gently to disperse grapes. In medium-sized bowl, combine remaining ingredients and whisk until smooth. Pour custard mixture over bread cubes. Stir gently, pressing down on any bread cubes that pop up out of the custard mixture. Set bread pudding pan into a larger pan, such as a 9×13-inch pan, or a roaster. Place doubled pan into preheated oven, then pour very hot tap water into the outside pan, filling it at least half way. (This is called a hot water bath and will make the pudding creamier.) Bake for 50 minutes to 1 hour. Top of pudding should be golden in spots and lightly puffed. Remove from oven, let cool slightly then serve. Top with brown sugar whipped cream if desired.