Originally Posted by Stretch

Drilling a straight hole under the car like that is difficult.

If you can get the hole to be the same size as the bolt needed to attach the shock (what is it M10?) you can use a longer bolt and a stiff nut on the other side of the nadgered threaded portion of the trailing arm. It is tricky to do this but I have managed to hold a nut in place with bent needle nose pliers stuffed through the coils of the spring.

The best option however is to remove the trailing arm and grind off the square threaded block and weld on a new piece.