You would be surprised how flexy they are. Even the body mounts play a role in keeping the body from flexing. My friend 75 K5 I'm working on, the doors would rub the rockers (doors sagging) with the old body mounts. He put a set of new mounts on and the body lines lined up again and doors no longer rubbed the rockers. Driving it down the highway the firewall/windshield frame would wiggle/bounce. The inner to outer rocker connection plays a big role on full tops to as it is what connects the floor to the pillars, so when that rusts out it gets floppy
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