Most often, it is not sediment which blocks the bottom of the water jacket. It is most often caused buy a stop leak product used over the life of the engine so that a leaky, and very expensive radiator doesn't need to be replaced. The tell is that it is hard as a rock and needs a lot of mechanical effort to remove all of it.
These trucks were work horses, many of them owned by poor farmers. Road hard and put away wet. They did what they needed to do to get whatever job they needed to get done before dark. They likely didn't have time to replace a radiator, nor have the walking around money to spend on a radiator.
