These kits called Battle boxes or torque box reinforcement kits.

They seem to very solidly address the problem on lower control arm side that faces the rocker arm.
What about the other side, the head of the bolt is still located by the paper thin sheet metal of the frame.

Is strengthening one side enough?

 

the issue of strengthening is only a concern on the outer side of the lower arms. Its a single thickness piece of sheet metal. However the inboard side is considerably thicker. Its less of an issue, but CAN still be reinforced by a GOOD welder