^Adrenaline cars are not guaranteed from what I know, but they should try to get you something similar. I rent from Hertz Adrenaline collection frequently and have never had them not have a v8 of some sort, but it may not be what you wanted. For example, Camaro SS is extremely rare, it's basically always a Challenger RT (hardtop reservation) or Mustang GT (convertible reservation).
Another alternative is to book through Enterprise Exotic collection, they have a v8 American Muscle category as well.