Im with Carl.
I've build truck bodies (not the cabins) and flatbeds on small and big trucks on my former job. Also repaired a couple of frames. sandblasted and welded in new C beams and ofcourse smeared the whole frame with tar again. You just have to figure out how much of the old C beam you should cut away and replace (from crossmember to crossmember) because just as crenwelge said, Welds don't flex.
I hope you understand what a mean because it's quite hard to explain in english when i've learned everything in Swedish!
