Best to straighten them first. Heat in a pot of boiling water for 30 minutes and clamp to a board to dry for a few days. You may have to repeat this a couple times, and us a shim to overbend them. Once straight, let sit for a good week before using. Sand the back flat and install with strong rivets, like Corby bolts.
Horn likes to move, and warping after installation isn't uncommon. Sand slowly and DON'T let it get hot while sanding and polishing.