I had a 285 tire mounted to my Sportsmobile rear door with the old style door-mounted tire carrier. For me the tire sat just a hair above the bumper so with any flex it would rest on it. I also mounted a very heavy box with a Honda generator and 3 2.5 gallon cans on top of it on the passenger side. Once, in a rather articulated off road maneuver (professionals only
) I took a pic of the van all twisted up. I looked closely at the pic later and realized my rear doors were WAY out of alignment with the body, badly sagging with the twist. However, I never had any problems. No leaks, no cracks, no wear, no issues with opening/closing. These Ford vans do twist more than you think but they can take a lot of abuse. DaveB is the only person I've heard of that actually has cracked the body on a van but he offroads like no one I've seen in an SMB.
Yo Dave, howbout posting some pics?