You have to add up all the weight (car, trailer, cargo). You will be close to max. tow rating. If the trailer has brakes and you are very careful you should be OK. With the V8 hitch and an trans cooler you should be good for 6700lbs. Take a short test ride and try an emergency stop where it is safe to see how it reacts.
You are going against manufacturers recommendations, but if the combined weight for the trailer and vehicle are less than 5000 lbs, you should be good to go... I think most of the 5.0L mustangs (depending on the year) were about 3400 lbs, plus a trailer weight of 1800 for the trailer, you would be exceeding the towing capacity..