Heat the milk, 1 cup sugar and 1/2 cup orange juice concentrate in a saucepan over medium-low heat until the sugar dissolves, stirring. Cool slightly. (It's OK if the mixture curdles slightly.)
Whisk the milk mixture, 2 tablespoons bourbon and the whole eggs and yolks in a large bowl; fold in the bread. Cover and refrigerate until the liquid is absorbed, about 1 hour 30 minutes.
Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish. Fold the chocolate chunks into the bread mixture. Spread evenly in the baking dish and sprinkle with sugar. Bake until a knife inserted into the center comes out clean, about 1 hour 20 minutes.
Meanwhile, make the sauce: Melt 4 tablespoons butter in a small saucepan over medium heat. Add the remaining 1/2 cup sugar and cook, stirring, until lightly caramelized, 5 to 6 minutes. Whisk in the remaining 1 cup orange juice concentrate, 3/4 cup bourbon and the salt. Cook until thick, 10 to 15 minutes. Stir in the cream and simmer until the sauce coats a spoon, about 10 more minutes. Slice the bread pudding and drizzle with the sauce.