On an 84 month loan, you will be upside down for at least 5 years on the Mustang.
As a stockbroker / financial adviser for the past 30 years, my advice to my clients is that if you cannot afford the payment for a car on a 36 month loan, you cannot really afford to buy that car (and should really consider something cheaper).
This. Always be in a position to save money.
Work hard now invest in things that will make you money.
Toys come later