Try this link:
This should tell you all you need to know about your new truck.
In a nut shell, the max trailer weight can be anywhere from 2840 lbs. to 5680 lbs. with a 4.0L, and the combined vehicle and trailer weight rating can be as high as 9500 lbs. This all depends on the transmission, and rear axle ration that your truck has.
I would venture to say that no matter which combo you have, towing a Mustang on an average open car trailer shouldn't be a problem so long as the proper hitch and trailer brakes are used.