Start with the pecans. In a small saucepot, melt the butter, brown sugar, corn syrup, and cinnamon until the granules dissolve, about 4-5 minutes. Remove from the heat and stir in the vanilla, then the pecans. Set aside.
Lightly grease a 9 x 13 inch cake pan or casserole dish. Pour 1/3 of the pecans onto the bottom of the dish and spread with a spatula.
In a large bowl, whisk the brown sugar, maple syrup, pumpkin, milk, cinnamon, ginger, cloves, all spice, bourbon (if using), vanilla, and the eggs. Working one slice at a time, dip the brioche into the custard, letting excess drip back into the bowl. Place into the prepared dish. Repeat with the remaining bread slices, then pour the rest of the custard mix over the bread. Dollop the rest of the pecans overtop.
Cover the dish and let sit 30 minutes, or up to overnight in the fridge.
Preheat oven to 375 degrees. Bake the toast, covered, for 30-35 minutes. Uncover and bake an additional 15-20 minutes until the toast are lightly toasty and the pecans golden. Let cool 10 minutes, then slice and serve!