Preheat oven to 375 degrees. Brush a standard 9-by-5-inch loaf pan with butter and dust with flour, tapping out excess.
In a large bowl, whisk together flour, sugar, salt, baking powder, baking soda, allspice, and nutmeg, 30 seconds. In another bowl, whisk together butter, pumpkin, eggs, orange zest and juice, and vanilla until smooth. Make a well in flour mixture and pour pumpkin mixture into it. Stir together just until combined and no dry flour remains (do not overmix). Transfer to prepared pan and sprinkle top evenly with pepitas, pressing lightly to adhere.
Bake until a tester inserted into center comes out clean, 1 hour to 1 hour, 10 minutes. Let cool in pan 10 minutes, then transfer loaf to a wire rack and let cool completely. Bread can be stored at room temperature, wrapped in parchment-lined foil, up to 3 days.