Finding an almost rust free 67-72 cab here in Pa. is about the same odds as getting struck by lightning. But I found one. Check out this hack job on one of the finest cabs I've ever seen in our junkyards. This truck is virgin and even had the original working T-3 headlights! Low mileage but box is missing in action and someone bought the doors and seat. (I'll just bet them doors were rust free!)
This is a 1968 C-20 that has (had) a beautiful cab. The floor are rust free, it still has the original rockers and cab corners all of which are rust free with on holes to be found, and wouldn't you just know it, some knucklehead hacked the whole rear section out of it! This could have another section welded back into it I guess...
In the first pic, we see the back. Rear window was removed and a huge opening was cutout, then the rest of the window area was patched over with sheetmetal and screws.
The second pic shows this horrifying "monster garage" hack job from the inside. Note the floor. I've never seen a rear floor with no rust/holes before.