DARLINGTON, S.C. (AP) — Brad Keselowski can’t wait to get to Darlington Raceway — and it’s not just because he won the last time NASCAR’s best showed up. This the fifth year the Southern 500 is NASCAR’s throwback weekend, kind of an Old Timer’s Day for fans and competitors to honor and mimic the greats of the past. That includes bringing back splashy old paint schemes, vintage uniforms and…