What keeps these frames from shifting?

[LostMy65]

[Captainfab]

The front suspension crossmember does the most IMO, with it's width and the number of bolts. The trailing arm crossmember and the rear shock crossmember have a spread of a few inches between the bolts that help with keeping the side rails in check. Just my .02

[LostMy65]

Just looking at a naked frame, and thinking of the lateral wall bracing required in construction started me thinking of angle bracing we use to resist lateral forces.
Makes me think of the x-frame of the 60-62.

[Captainfab]

THAT'S the word I was trying to think of......lateral. Yep the '60-'62 frames are a bit stiffer than the '63-'87 frames.

[LostMy65]

I guess once the cab and front clip and bed are all mounted it's like a solid piece of plywood on a wall.
I just looked at pdxhall's naked frame and thought there isn't much there to resist things getting knocked out of whack.

[old cruiser]

Not knowing but only guessing , the earlier frames also have different suspension and these frames using the suspension of coil springs etc may also rely on some flex of chassis for tat smoother ride GM may have been chasing. Its possible that these later chassis may have been made of different quality steel to allow for above. As I said I don't know just guessing.
Holden's down here did this in the 70's and called it radial tuned suspension