[leegreen]

I'm with Mr48: metal fatigue caused that curved crack, even if the original weld followed that same curve. I'd look for reasons that the lower control arm would be bending the ends of the cross member front-back before going further to just weld it all stronger.

For further reinforcement, a plate that includes the hole and ties to the new lower plate on the cross member, as Mr48 shows, but taper it along the crossmember to avoid creating a stress point
Reweld those cold welds between the new lowerplate and cross member,
they are not critical welds but they are also the first welds any inspector looking under the truck will see.