Roast Goose with Pork, Prune, and Chestnut Stuffing

1 cup pitted prunes
1 cup brandy
7 tablespoons unsalted butter
1 cup minced onion
One 10- to 12-pound goose, liver chopped (see Note)
1/4 cup ruby port
1 pound fatty ground pork
2 large eggs, lightly beaten
Generous pinch of ground allspice
1 teaspoon chopped thyme leaves
1 garlic clove, minced
Kosher salt
Freshly ground pepper
15 ounces cooked and peeled chestnuts, coarsely chopped
Boiling water
3 cups chicken stock