If it's the side (web) of the frame that's rusted through, it can be repaired. Many truck frames have a strong recommendation against welding or drilling on the flanges. But the older trucks weren't made of alloy steel with that issue. Doubler plates (you can fabricate your own) welded in can deal with smaller issues in the frame. I repaired a couple spots on my frame (rust through where the bed wood contacted the rear axle arch, and one of the rear bumper mounts.)
Post up a picture of the damaged spot.
