Question for you since you have seen more accidents that the normal person would care to think about. I have seen a few people running harnesses in non caged vehicles. To me having your body stuck in the upright position in a roll over doesn't seem smart. Being able to lean whether voluntarily or by the forces of a roll over would seem to help when the A pillar is crushed. Is this how people are surviving these type of wrecks?
I have never seen one with belts like that involved in an accident, but looking at them I see that some do not attach the shoulder belts anywhere high on the seat back. This would not keep an occupant upright and seems more of a fashion statement than having any real protective quality. The factory seats we have are not designed to take those kinds of forces in the seat frame, and would need to be replaced with real racing seats in order for proper shoulder belts to have any actual effectiveness in a crash. Not just a fender-bender, but a real
wreck.
And trust me,
nobody is strong enough to voluntarily control their body in a highway speed crash. That's why racers have HAANS devices and arm restraints and helmet restraints and leg restraints and on and on... It's just physics again. A bad wreck causes so many G's that there's no way to fight it and you just get hurt worse if you try. This is just my own belief, but I think the reason why drunks walk away from wrecks that would kill sober folks is that they don't see it coming and are relaxed when it happens. Their reactions are slowed to the point that they don't stiffen up at any point during the crash, and then they climb out of the wreckage going "What just happened?"
I think the reason folks walk away from 80's rollovers where the A-posts give is good engineering coupled with a healthy dose of good luck. Those A-posts are
working when they bend,
absorbing energy that might otherwise be directed into your body. The same with the rest of the truck. Newer vehicles are designed to be sacrificial to protect the occupants just like biltforme pointed out, and these 80's were built just when that design philosophy was starting to be put into play. It truly is shocking how much force is generated in a highway speed accident, that much weight going 60 miles an hour has the energy equivalent to several artillery shells detonating simultaneously. That's a

-ton of energy, and it all has to go somewhere. It's even worse when two opposing cars hit, the total energy amount increases sharply with the speed and it all gets converted from kinetic energy into other forms.
Some of that energy is spent as heat, some as noise, but most is used up bending and breaking stuff. The safety engineering is trying to make it so the car bends and breaks and not the occupants. And they do one hell of a good job these days, folks are walking away from wrecks where you have a hard time telling what
color the car used to be, let alone what kind it was. Used to be if the front bumper was touching the front wheels somebody was almost surely dead- or would be pretty soon. And in the 60's and early 70's cars this was a fair assessment. Engines came back through the firewalls, steering columns did not collapse, doors flew open, dashboards and interiors were all metal, and seat belts were poorly designed and hardly ever used. A very different world than in today's vehicles where the entire engine can be missing and the front wheels laying in the street and you still have a living patient who will very likely survive with little or no long-term deficits. Amazing, really.
These trucks were designed and built in the 90's, where automotive safety design was undergoing some pretty radical rethinking. Some made it's way into these trucks and more into later models, but they are significantly safer than earlier models. If Booger's wreck had been in a 40 you would have seen a very different outcome.
I think the take away from this should be that despite the pictures, look at the outcomes. Most folks walk away from accidents in 80's series LC's, despite what the truck ends up looking like. These rigs do the safety side of their job very well, just like they do everything else well. They
do protect their occupants and some of that protection happens by destroying itself and absorbing the energy that would otherwise be absorbed by comparatively fragile human bodies. The pictures
look pretty bad, but that's just the truck doing it's job. How many folks walked away from those wrecks? Betcha almost all of them. (Not including that side impact one though, that one would have been real bad no matter what the vehicle was.) You're pretty safe in an 80.
Hey, I drive one.
Hope this helps.