I would bolt it on with grade 8 bolts like was said. There is a reason it was riveted and not welded originally. It needs to flex a little and welds will over time crack.
The reason was economics. Rivets were cheaper than welding processes. That and welding on the flange of a frame weakens it, which is what would be the reason for cracking. Welding should only be done on the vertical section of a frame, and judiciously if any.
