Trucks have always had a front half & a rear half. I can tell by the ride & handling, feels like there's a hinge in the middle. What if the bed & cab were joined at all 4 corners by very large bolts, spacers & washers? I remember reading about the Honda Ridgeline when it first came out. Body & cab are one piece & testers said it rode & handled like a car. There's probably a reason trucks come in 2 halves, otherwise they wouldn't build them that way. But I'm a tinkerer. If it ain't broke, experiment. Just a thought, not sure I'll have time before the New Year.

 

Rancheros and el caminos had bed and cab together

Ridgeline is uni-body which has lower tow capacity than body on frame construction does

The reason most trucks use bed as a separate part is so you can put on a flat bed, or ???
So manufacturer can make the bed what you need to, to sell it to companies or for people to build to suite, i.e. install utility bed with all the doors and storage

That started back in the early 1900's with the first "pickup trucks" and I think some of the first were Dump truck conversions, so needed a separate bed

With off-roading you probably wouldn't want, unibody, or a long body on a long frame, as frame twisted a bit it would damage the body

 

Interesting, thanks for the info.