I don't know when they started adding a little steel to the frames, but I cracked several mid-late '80s Ford frames from on running gravel roads at 50-60 mph. At the time they weren't made for the gas shocks I ran and the steel around the mounts would break out.Back to the broken frame. I had a 97 F150. It had the tow package. I got it used. It had a 2 5/16 ball on the hitch. I removed it and installed a 2". After 5 years of heavy use, and I did overload it a lot hauling stuff to build a house, I had a horse trailer on it. Drove 80 miles to a vet with horse, drove back, unloaded horse. When I went to unhook the trailer, the back of the truck just kept going up. The frame on the truck had broken almost in to on both sides. Just where the rubber bumper from the bed was hitting the frame. It most likely had been cracked for years. Only the top section of the frame stopped it from coming completely into. I jacked the middle back up, unhooked the trailer and pulled it out of the way. With the middle jacked up, the crack in the frame closed up. A friend welded it with plates. Drove it another few years. Lucky, I didn't show up on the news with a wrecked truck and trailer with dead horse.
IIRC, they used the same frame for the F250 & 350 as they did for the lighter trucks... and still had less payload than many half tons today. They also were mostly regular cab and weighted about 2000 lbs less.