Sixt also has some Mustang types for rental. The lowest rental rate you`ll get by using the
Sixt deal page. This is an option for around 20% reduction.
I suggest trying to return by Monday before noon if you need the car over a weekend @5.psi. Reason behind is that weekend rates end at monday noon and are much cheaper than any rates during the week.
An issue always is of course that usually car rental companies guarantee a category only and not a specific car type. Specific car types to be guaranteed are more expensive if there are any.
Personally I would reserve the full size category. When you are there you can ask for a Mustang. It might happen that the yask then for a small upgrade fee if they want to make some additional money.