Adjust oven rack to lower-middle position and preheat oven to 350 degrees. Line two (2) 9-inch x 5-inch loaf pans with parchment paper. In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon, pumpkin pie spice, and salt. Set aside.
In a large bowl whisk together sugar, pumpkin, oil, and eggs and mix well. Add dry ingredients in 3 batches, mixing well after each addition and scraping down the sides as necessary.
Divide batter between the two loaf pans and smooth the top with a spatula. Bake 1 hour and 10 minutes or until a toothpick inserted comes out clean with a few crumbs attached, rotating pans after 30 minutes through and checking for doneness after 1 hour.
Cool 15 minutes in the loaf pans on a wire rack over a baking sheet. Slide a knife around the inside of the loaf pan to loosen the bread and flip out onto the wire rack. Remove parchment paper and cool loaves completely.
Meanwhile, combine powdered sugar, and butter in a small bowl. Whisk in the milk, 1 tablespoon at a time, until slightly thick and smooth.
Pour glaze over the top of the cooled loaves so any excess glaze drips through the wire rack and onto the baking sheet. Garnish loaves with pepitas and allow glaze to cool and harden, about 30 minutes.