I know that they are pretty stiff to bend even after 60+ years they should be tight enough that with a load on them they barely move. I figure they have to have engineered them figuring 200lbs or more per corner and then bumpy roads making them move straight up and down since they are rigid on the front mounts. They were changed in late 1948/early 1949 because the center mount was causing stress cracks in the cab. Same reason for the change in radiator mounts from one center mount to two spaced a few inches apart.

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr