deepwater blues

I'm researching a 1991 Lariat XLT with a 351 5.8 L, 5-speed auto with tow package. The owner doesn't know what the tow rating is and I cannot find anything specific to this truck. Does anyone know the rating for this truck or know where I could find the information? I will use it to tow a tandem trailer with boat; combined weight: about 7000 lbs, terrain: hilly in some areas, paved roads.
Thanks.

Also posted this in the area specific to year 1991.