yup--night and day!
We were often amazed at how good the chassis was compared to the sheet metal too,after stripping several GM trucks that were rotted to dust!..often the frames actually had some paint still showing in places!..my 79 C10 has a nice frame,still painted in most places,no rot at all! --how I wish it was a 4x4!
My 74 K20 HAD a nice frame when I got it in 91(frame was a 77 actually!)
I even painted it with asphalt based chassis paint when I first got the truck--it lasted 5 years,then it fell off in sheets like saran wrap..when I cut it up this summer,the body AND chassis were spiltting apart like delaminating plywood!--I could push a screwdriver thru the frame,and break off big chunks in my bare hands!.only where the plow pump and transfer case leaked was mint!--the rest was a shame..
The 73 k10 frame I just cut up for scrap sat under pine trees for 10+ years--as I cut it up,I felt bad,thinking I should have put my K20's axles under it,and kept it..but the cab was patched 3 times over,and rotted to death again..also thought of saving and using the spring hangers,etc,to convert my C10 to 4wd--but I am in not in good enough health to do that kind of a job anymore--I've had enough..done it too many times already,and seen too many rot away in record time after all that labor..very discouraging
It would be different if they lasted 20 years,like if you lived in arizona..
The calcium cloride road salt they started using here in the 90's really EATS steel bad!..I don't remember rock salt being so caustic,it used to take years for steel to rust ,now it takes only a few months after being exposed to the "new" road salt..I'm all for safe roads,but not if my truck dissolves in 5 years!(brake and gas lines only last 2 years here now--even newer 90's trucks are popping brake lines already--they must use recycled steel or something!)..
I heard GM's plants were flooded in detroit in the early 70's,and many rolls of steel sat submerged in water in the basements of the stamping mills where the body panels were made..I beleived it--the 73-75 trucks rotted faster and worse than any other years ,despite the body style staying the same..many guys who bought a "new" gm truck due to the new style body and liked the looks of them were disgruntled a few years later,when holes were appearing above the rear wheels already...
One girl I dated in high school had a father who bought a nice 73 K20 brand new--I remember him letting me take his daughter to the movies in it--(he didn't like her riding in my 56 Chevy 3200 pickup much.)..I recalled his tail lights being held in mostly by duct tape,his rear quarters were rotted so badly already (this was late 75!)..I joked my 56 had less rot than his did!
and the frame on my 56 was 3/8" thick too--at least twice as thick as any GM truck 1960 and up I've seen..they don't make 'em like they used too..
The '75 K5 has a little rust, but that was from the ONE winter that I drove it in Massachusetts. 
I don't miss the evil road salt at all! 
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