Since this is rare the Manufacturer might be interested enough to help you... this might be a "just in time" part from one of their vendors.
I know a bit about welding and worked where we had heat treating facilities and all welders were highly certified, all welds were xrayed and die checked along with other tests.
I would be uncomfortable with any welding on the frame even to repait it... and drilling is probably not good either.
If you repair the broken frame and trade it off an unsuspecting someone is going to be put at risk... and that's not good.
I personally think if the frames metal is crystalized enough to break it will probably break somewhere else if you continue to use it even with U Bolt clamps and shims.
I doubt that a junk yard would have a useable frame... I can't imagine a truck that is bad enough to be junked that has a structurally sound frame.
I would contact the manufacturer and ask their opinion.... and help.
Good luck
Sleepy