I don't know what you would gain from welding the pits, you might do as much damage or more by the heat. It would be a tough call with out being there and getting to see it up close. Have you thought about some of the "Por" products. They are a rust encapsulator type paint with a lot of solids in it. If you plan to really use this truck for heavy loads, reinforcements from the inside might be the way to go. Sandblasting will be the first step no matter which way you go. Get it clean for a good look. Joe