You might consider posting this in the eHam Mobile Forum. Lots of good experience hanging out there, like Alan, KØBG (www.k0bg.com
I saw a ham once who cut a piece of marine grade 3/4" plywood to fit the bed of this pickup. He mounted the antenna to a small TV style tripod bolted to the center of the plywood and "guyed" it to small eyebolts at the corners of the plywood. It did not slide around and the antenna mount was solid. It sounds like a monstrosity, but really was not that hard on the eyes.
I couldn't find any similar approaches on Alan's website, but there are many installation options discussed there. Also, give some thought to the plywood foundation. You could easily build handles on the base for removal as necessary. Just a thought to get the old noggin' cranking.