Hooked,
this has been a big topic for a long time, even in the latest Mustang Monthly they discuss this issue. Why...because all antennas came from the factory in the trunk and were installed by the dealer. They did not always follow the template in placing it and if it got misplaced in the prep-work they just grapped one in the inventory.
Now to your question, everything I have ever heard about the '68 antenna is it should be round to be factory correct, NOTE I did not say DEALER correct because you could have bought your car with a dealer inventory one not factory.
As a sidenote, Mustang Monthly stated that some dealers ordered cars without radios (and of course there would be no antenna) and then put in what the buyer wanted, that way they did not have to guess what was wanted and the dealer installed radio kits (radio, antenna, and bracket) cost more so they made a little more profit on the deal.
Doug