How much frame shows under the running boards depends completely on how low you place the body on the frame. On your suburban you may have to choose between how much rear floor you want to lose and how low you want the body over the frame.
an 8" wheel with 3.5" backspace fills the wheelwells quite well, while leaving enough room for turning. In the back 15x10" with a 3.75" backspace fills it good, but that's on a pickup and with tubbing the box. You might have to play with it on your setup.
