I spoke at length with Dewey Walker from Lance and here is how he explained the Lance 650 / Ford F250/350 issue. The Superduties have a higher cab roof than other trucks. It’s necessary to install a platform to raise the 650 about two inches off the truck bed for a safe two inches clearance above the cab roof. Dealers know how to do it, he said. Dewey carries his demo 650 on a F350. I questioned the raise in vertical COG and was told it was mimimal. This raise would also solve my problem of my fifth wheel hitch rails left in the bed when I remove the 5W hitch.
In the course of the conversation Dewey recommended air bags over SES, saying the big double bags would provide lateral stability.