Give it all the tries that you want to, DrivingRain. I always book fullsize and almost every time end up with the Taurus unless I want to trek back to the office and talk about it with an agent who may or may not know the Centurion policy. When Hertz says "subject to availability" on any promotion, what they really mean is that they won't give it to you automatically even if they have twenty unreserved Crown Vics and Town Cars on the lot. You need to ask for it in person.
They do however get one Centurion perk right automatically and that is the immensely valuable four hour complimentary overstay. I've never needed to argue about that one.