1. MAKE THE CORN BREAD: Preheat the oven to 425°. Pour the oil into a 9-by-13-inch baking dish and heat in the oven. In a large bowl, whisk the cornmeal with the flour, sugar, baking powder, salt and baking soda. In another bowl, whisk the buttermilk with the eggs. Pour the buttermilk mixture into the dry ingredients, add the melted butter and stir with a wooden spoon until just blended; do not overmix.
2. Remove the baking dish from the oven and swirl to coat with the oil. Scrape the batter into the hot dish and bake for 25 minutes, or until springy and a cake tester inserted in the center comes out clean. Transfer to a rack and let cool. Turn the oven down to 375°. Crumble the corn bread into small chunks and spread on 2 large baking sheets. Bake the corn bread for 15 to 20 minutes, stirring occasionally, or until dry and golden. Leave the oven on.
3. MAKE THE STUFFING: In a large pot of boiling salted water, cook the collard greens until just tender, about 5 minutes. Drain and let cool slightly. Squeeze dry and coarsely chop.
4. Melt the butter in a large skillet. Add the chorizo and cook over moderately low heat for 5 minutes. Add the onion and celery and cook, stirring, until softened, about 10 minutes. Stir in the collards and remove from the heat. Let cool completely.
5. Scrape the corn bread into a very large bowl and add the collard mixture. Season with salt and pepper. In a bowl, whisk the eggs with the stock and pour over the stuffing; mix well.
6. Butter 2 large, shallow baking dishes and divide the stuffing between them. Cover and bake at 375° for about 25 minutes, or until heated through. Uncover and bake for about 10 minutes longer, or until browned. Serve hot.
Make Ahead: The corn bread can be frozen for up to 1 month. The stuffing can be assembled and refrigerated overnight. Bring to room temperature before baking.