Preheat oven to 350° (170°C). Spray 2 (9x5-inch) loaf pans with cooking spray. Line pans with parchment paper, letting excess extend over all sides of pan.

In a large bowl, whisk together flour, sugar, baking powder, salt, and baking soda.

In a medium bowl, whisk together buttermilk, oil, eggs, and vanilla. Fold buttermilk mixture into flour mixture just until dry ingredients are moistened. Fold in puréed peaches and 1 cup (113 grams) pecans. Divide batter between prepared pans (about 3¾ cups or 1,006 grams each), spreading smooth.

Bake for 30 minutes. Place peach slices on top of batter, and sprinkle with remaining 2 tablespoons (14 grams) pecans. Bake until a wooden pick inserted in center comes out with a few moist crumbs, 45 minutes to 1 hour more, loosely covering with foil to prevent excess browning, if necessary. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks. Store in an airtight container for up to 3 days, or tightly wrap in plastic wrap, and freeze for up to 1 month.