If it wasn't clear in the last picture, this view should help. We're looking from inside the trunk, with a section of the floor and wheelhousing removed. You can see the centerline and just about make out its corresponding line on the top of the chassis. This is the centerline of the axle. Here Matt Proctor is measuring the outside width of the C-notch. It should be 3/8-inch wider (taking into account the two 3/16-inch plates) than the chassis rail width, but it never hurts to check. The sides of the 'rails may not be exactly vertical for instance.