In my owners manual (2005 F-150 SC), you must deduct 500lbs. from the rated tow capacity if you have 18" or 20" wheels.

 

I've towed with both. A 1998 F150 Lariat with a 5.4 and 3:55 gears and now with a 2006 F150 Screw KR with the 5.4 and 3:73 gears. I tow a 22' Grady White boat when fully loaded with fuel and equipment weighs in at 6000lbs, my 98 towed fine and that year only had 235hp, as far as pulling and passing comfortably I had no problem, the only thing I didn't like was the factory brakes. My 06 with 300hp does a better job but I wouldn't say it's night vs. day, braking with the 4 wheel discs is much better. I would not have any reservations about towing 7000 to 8000lbs in my 06. With the exception of the occasional trailer hitting a pot hole I hardly knew it was behind me. The owners manual says my truck will handle up to 9200lbs, I'm sure with the 3:55 your still up in the 8k range. Good Luck.