If you ate out of luck for Shelby availability at the counter, at LAS there will be interesting upgrade choices from the ultimate choice cars.
in the past, Range Rover Discovery Sport, and Mercedes C class AMG could be spotted.
if all that fails, Chevy Camaro SS is an exciting toy (but only the SS version), and a cheaper upgrade.

I assume that you are traveling to the US from abroad. This means the rates you get include full insurance. If you select premium car classes, the pricing gets ridiculous with that insurance.
However, when you negotiate an upgrade, the insurance for your original full size reservation will remain and cover your ultimate choice rental.