This is a frame from a 1959 Chevy Impala Convertible. These photos illustrates the differences between a convertible frame and a standard frame. Most notably there are additional steel channels welded above the rear arches, plus a cross brace just ahead of the rear axle. The convertible bodies were mounted to the frame with no rubber cushions, but used steel shims instead. I am not sure of other differences.
x-Frame Chevrolet cars and parts for sale on eBay!