Don is right, in the correct way to figure, and that many with 350 diesel trucks, tow up to 14,000# FWs.
However, if you want to stay under your payload rating, that is right on the door jam tire loading sticker. Most important, will be your rear axle rating, and tire load rating.
To make it easy, if your truck weighs 8,500 loaded, including a 200# hitch, and your truck GVWR is 11,500, you will have 3,000#s left over for FW pin weight.
That would be a FW with about a 15,000# GVWR
This is just an example, to show easy figuring.
Jerry