The f350 with 20s will be high, if I'm not mistaken, 350 rear blocks are usually 1 inch taller. My 06 250 with 325/65/r18 is (34.6 inches) the same diameter as the 20 inch wheels and tire combo that ford uses, and the 350s sit right at an inch higher when sitting next to my truck. When my Eagle 339flqs is hitched with the B&W companion, it sit nose high. I am raising the pin height this coming weekend. If yours has the Mor-Ryde pin box, it is adjustable as well. Have you checked it yet?
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2016 jayco eagle 339flqs, 06 F250 CCLB, STUDDED and FIXED, Pmax turbo, geerhead tunes, B&W Companion, Firestone bags