Drill the spot welds out and take the rear quarters off the box sides and you'll find the rust on any of these trucks. They had no internal galvanizing or paint on the hidden substructures, so even when you find a good looking truck that seems to have no rust and no holes, it's there and it's started .I put a mirror up inside my 85 w-150 and its the same. A friend of mine has a never winter driven 92 cummins holiday truck,the rockers and cab corners are out of it.
My ramcharger was a pretty clean pacific northwest truck with a few rust blems. When the bodyshop went digging they found the internal metal uncoated the surfaces covered in rust and some of the structural eaten away. The tub was disassembled, blasted, welded back together and epoxy coated.