The 1999 F250 can tow any where from 6900 to 9700 depending on if it's a 4x4, Crew Cab, Super Cab, automatic, manual, trailer Tow Package,
You see where I'm going, actually NO you shouldn't be towing it becasue the GVWR of the TT is >14k
This is the number that will make the difference. Your truck is not set up to properly do the job safely. Not only that you will most likely not like the way it tows with that engine. If you are luckly maybe 6-8mpg down hill on a calm day.
Personally it's time to go diesel, 350 or 450, better fuel mileage and a better piece of mind pulling the load.