Wayne, the specifications page for the fifth wheel will give the "Pin Weight" for each of the models. This can range from 800 pounds up to over 3000. You need to be concerned with the pin weight (which will add to the "Payload" of your truck) and then the total weight of the trailer, which will be the amount your truck is rated to pull. The 250 and 350 with diesel are both rated at 15,000.
This should answer your questions:
http://www.fleet.ford.com/resources/ford/general/pdf/towingguides/Ford_Linc_16RVTTgde_r3_Nov12.pdf