Your current exhaust system comes through the lower panel below the bumpers. You will want to have those holes filled, or purchase a new panel and paint it. Nothing looks tackier than the mid year with the old panel with empty exhaust holes and side exhausts. When I put the side pipes on my 427 1966 convertible in 1967, I had the holes fiberglassed over. I don't remember having to do any cutting during the installation of the side exhaust, but I am losing my memory on some things that I just always thought I would remember, expecially dealing with my corvettes.
Barrett