In `68, the Super Sport option was RPO Z03. It could be ordered on any Impala, regardless of engine choice. Essentially, what this got with RPO Z03 were Bucket Seats and Console, floor shift (both auto and 4-speed), SS specific door panels, and Super Sport badging on the Dash Board, Door Panels, Grille, Trunk Lid, and front Fenders.
If you have a build sheet, or any other form of original documentation for the car, it should indicate RPO Z03. If it was a Canadian built car, it will show RPO Z03 on the Cowl Tag. US built cars will not show this on the Cowl Tag.
Hope this helps.