As long as you aren't exceeding your tire or axle weights with all the pin weight of the FW then you can go as heavy as you want. Tow capacity is not a legally enforceable issue, that is, no one's going to ticket you for towing over the number you found in the manual.
The heavier you tow the slower you will be going up hills. That can be addressed by modifying the truck and transmission for more power or you can just live with the slow ascent.
You also need to pay attention to your state's tag weight laws. If your truck is only tagged with passenger car tags rated 7,500 pounds and it actually weights around 12,000 pounds hitched up you could get a ticket for that.