Sometimes it's difficult to reserve the car you really want, especially far in advance, and also very close to your pick up time.
You can call Hertz reservations and ask for a specific car, telephone reservations can do things we mere mortals can't do using the website. I would call reservations and ask for a prepaid (WOW) rental for your car.
You can book the Mustang convertible, if it is not a guaranteed Mustang booking, Hertz may switch it for the Toyota Solara or Sebring convertible. Which are obviously not as cool for South Beach.
Hertz has the Corvette (miss the Cadillac ZLR and Jaguar XK8) in its two seat category and the Audi A4 in a 2+2 category. If you see a car in the Hertz lot that you like and it is not already rented, there is nothing to stop you asking how much it will cost to upgrade, even if your rental contract is already printed. The reason I say "ask how much to upgrade" is because if you just ask for an upgrade you are unlikely to get it, because the agent will think you are demanding a free upgrade - and that get's you off to a bad start in the negotiation.