A truck box frame includes first and second laterally extending frame members and a frame side rail assembly. The frame side rail assembly includes first and second frame side rails. The first and second frame side rails generally extend between and are coupled to the first and second laterally extending frame members. The first and second laterally extending frame members are extendable laterally, creating a variable width truck box frame. The first and second frame side rails may be curved or swept so that the intermediate portions extend inwardly toward one another, forming a tapered portion of the frame side rail assembly. The length and curvature, or sweep, of the side frame rails may also be varied to create a variable length truck box frame.