I used Auto Rust Techincians's (autorust.com) frame connectors on my challenger. They aren't supposed to be through the floor, but on my car, i still needed to slit the rear floors for them. They come up level with the floor and are completely invisible under the carpet. They are made with 2x2x.125w steel and are incredibly strong. They are stronger than the floor-contour units. A 2x3 would be even stronger.