I did a survey back in 1985-90, and round antennas were on 98% were on those GT/CSs.
I would go so far to say that ALL GT/CSs had the round antennas.
But, for the folks with a square one that looks "right', consider that your antenna might have been broken in 1969, or '70, and the dealer put on the service replacement version, which was the square based version. Then, that part aged along with the rest of the car over the past 39-40 years.
All the advertising art and photos taken of that period showed round based antennas on the cars.
This doesn't mean that IF you have a square based antenna that you'll be busted by the "Concours Police"...LOL....but that what I know from my survey and research, it's overwhelming that they used round ones in '68.
(there may be an anomaly--that the New Jersey built cars (non-CS) used a supplier that only provided square based antennas. Maybe someone knows this?? Mr. Speegle?)
Do what you wish...it's your Mustang.
Paul N.