The metal? like the fender part, not the flare part correct? ouch. time to update to some flattys! :P haha jk. but not a bad idea if you like the look.
I imagine take a hammer to the underside, maybe place a board on eitherside, and clamp then hammer to make it level. then you might want to do some paint, or at least some clear coat to protect the area from rust. pics would help.