To find the exact towing capacity for YOUR 1999 F-150, you need to:
1. Locate the sticker on the driver's side doorjamb. This sticker will show the GVWR (Gross Vehicle Weight Rating), the GAWR (Gross Axle Weight Rating) for the front and rear axles, and most importantly, the GCWR (Gross Combined Weight Rating). The GCWR is the maximum weight of the truck *plus* the trailer it's towing. This is the most relevant number for determining towing capacity.
2. Check your owner's manual. Your owner's manual should provide specific towing capacity information for your truck's configuration.
Without knowing the specific details of your truck (cab style, bed length, 4x2 or 4x4, and any additional options), it's impossible to give a definitive answer. However, you can expect the towing capacity of a 1999 F-150 4.6L V8 with a 3.55 axle ratio to be somewhere in the range of 6,000 to 8,000 pounds, but this is just an estimate. Always rely on the information found on your vehicle's sticker and owner's manual.