Went to miami about 8 years ago now I done exactly that. Booked a convertible mustang months before I left when I got there there was no mustangs available,, as people rent them for a few days then just don't bother taking them back, I had the right hump and kicked off they upgraded me to a 350 E class that was fully loaded with double panoramic sunroof, which by the end of the holiday I was actually glad about it as the roads in miami and florida are absolute dog s**t.
Just be aware though you might book a mustang but it might not actually be there when you land