The sport coupe has the fastback styling but GM never called it a fastback. Since the Custom Coupe was new for 1968, I would not call it the standard. Remember, beginning in 1965 the "fastback" was the only hard top body style available on Impala. That's what people were used to seeing. The caprice was only available as a 4 door in 1965 but in '66 - '68 it had the Custom Coupe roofline. I guess people who wanted the caprice roofline but couldn't afford the premium price now had an option.