In the case of THIS truck, I wouldn't think that its /6 would have enough power to "hurt itself"-- relating to body and frame flex....

It's not just the engine that plays into it. It's the roads you drive, the loads you carry. That steep driveway you pull into at an angle without giving it a second thought, could be flexing things quite a bit (just saying as an example).

I've had a '76 and an '84 that had frame issues. The '76's issues were brought on by mileage, the '84's issues were possibly caused by a (very) minor accident the truck had been in early in its life. In both cases the frames had zero rust and neither truck ever carried something really heavy (especially the '84).