The F-350 if you can get it with the 4.30 gears has a maximum tow capacity of 15,000 lbs. Without the 4.30 gears it has a 13,100 towing capacity. You can see if the F-350 with the gas engine can be special ordered with 4.30 gears.
The alternative is to go with the diesel engine and either 3.73 or 4.30 gears depending upon the platform, F-350 or F-450.
With a 14,000 lb. trailer I would also want the downgrade braking provided by the diesel with its exhaust brake and not want to be relying 100% on the trailer's brakes.