The NOS K frame deal was tongue in cheek no doubt its cost prohibitive especially for a hemi E body K................

It was just an observation of 40+ year old abused pot hole ridden street use vs a pristine example that has had no stress at all.

Just like the cars unibody structure the 40 + year old high mileage example will have had stress applied to it vs a virgin tub.

Trust me spot welds do eventually fatigue.

I worked at a Borg Warner plant that made transmission bands the spot welds on a few select welds were tested via destructive testing every shift. They know how to do QC in this century, back in the last century Americans laughed at Demming.