My plan is to hire an Intermed' with Hertz Gold for the 3 days (June 9th), and then do some chatting with the Gold desk to get the best upgrade price for any car in the Adren range...
And yes, I heard about the Shelby Mustangs, will be interesting if they do have availability and the upgrade cost.
CJD - As I book with Hertz UK, it comes with full insurance, no excess, etc, unlim miles (not that I would do more than 300 miles in 3 days anyway!)

Just hoping theres some nice Adren cars available, and Hertz don't have a complete tool on the Gold desk !

Max $40 upgrade per day I say! Still want the V8 Challenger mind!