Frame swap

[GMtrucknut!!]

I stopped and looked at a truck today that was for sale and the individual told me that his plans were to mount this 1967 Chevy shortbed on a 1999 chevy truck frame and stated that it had a much better frame.Upon inspection of the frame I noticed no damage the current owner then replied that all these trucks 1967-72 in particular had frames that flexed too much . My question is has anyone ever heard of this? I have worked on and modified these trucks for 30 years and have never experienced this ,am I missing something??