I don't have an informed opinion on your truck's tow capacity, but I do have a lifetime of towing various loaded/overloaded trailers with 1/2 ton thru my current 250,000 mile 3500 Dodge Diesel dually and driving a log truck in my very early years.
I wouldn't try what you're proposing on a trip type drive. In all likelihood the mustang weighs about 3,000 lbs., the trailer about 1,500 lbs. So, 4500 lbs plus 4,000 or so for the truck and you're looking at 8,500ish lbs. True, good trailer brakes will handle the stopping but the trucks suspension I'd think is the weak link.
The 216 is also going to mean driving at 30 to 40 mph a lot.
Now all the above said, I'd likely hook up to the trailer, pull it a bit and see how it does. If OK, I'd load the Mustang and pull it a mile or so and if I felt comfortable and I had a car show in my local town and it was only handfull of miles away....I might give it a go. But that's just me.
One thing about pulling loaded trailers, you need to always drive with your focus a long way down the road, when following other vehicles allow a "LOT" of stopping room and always assume ever car approaching a stop sign is going to pull out in front of you.
Good Luck,
Ron
Last edited by moparguy; 05/29/2017 4:11 PM.