To the best of my knowledge, there are no federal requirements other than to follow the manufacture's "Upbuilding" requirements. You can contact the manufacturer of the truck frame and purchase their "Upbuilding" specification. It will provide guidance for welding to their truck frames, i.e., how to mount dump bodies, boxes, etc.
If you do not follow the manufacturer's guild lines you are in violation of federal laws.
Generally, the truck frames are quenched and tempered high strength steel. It isn't advised to weld between the front and read outer most spring hangers.
I know fabricators frequently extend or shorten truck frames, but it isn't recommended. It is amazing how often things are done that are not recommended or allowed, but still it flies down the interstate at breakneck speed without mishap. All is good until one does fail and someone is injured or worse. Then all hell breaks loose.
One of my more recent court cases involved five new truck that were modified by the dealership. My client saw the modified frames and raised an issue. We got five new trucks as a settlement. I often wonder who the dealership sold the old frames to.
Best regards - Al