year and model would help. If it's an inside mount (1970-???) then put the pipe into the tank while mounting the tank (with the trunk floor gasket on it!), then once the tank is secured, work the pipe up to the mounting face and insert the raised part in the hole and get the screw holes lined up which install from the outside threading into the filler neck flange. You can easily tell if it is an inside or outside flange because the outside filler tube flanges have holes bigger than the screws and the screws thread into the body metal, where inside mounts have body holes bigger than the screws and they thread into the filler tube flange.
