I think there were changes in the metals or paints or something throughout the dent side era that affected how they rusted. Seems to me that the previous gen was about average for rust and then '73 was REALLY bad. '74 was better, 75-77 were actually pretty good, and 78-79 (and 80-81..............) were really prone to rust. Just my unscientific observations. I've worked on quite a few trucks from that era and have never seen one with ANY rot in the roof like that.
77-79s had a shit ton of galvenized steel on them. IIRC in 75 they did something to the inner fenders to help with rot in those areas. 73 and 74s were very rot prone.
67-72s seem to hold up well though. But i think they were thicker then the dents. I dont know a whole lot about the 67-72 "bumpside" trucks.
What doomed the majority of 80-82 trucks was the 100/150s used fords (in)famous swiss cheese frame in a misguided effort to save weight.
These things could fold in half if loaded anywhere near capacity in the right spot, let alone add rust to that.
83+ got alot better in that department. The 80-82s were a huge step down in quality and toughness from the 73-79s. The 83+ regained some ground but not enough IMO.
As far as im concerned 73-79 (Specifically 77-79) were the absolute pinnacle of ford pickups. Any pickup for that matter.