Thanks chaps.
Here's a pic of the box. The vertical rod A needs to be where the red dotted line is.
The weld is fairly chunky - cutting through it without damaging the rim of the silencer (or the rod itself) could be a bit tricky.
Unfortunately, the bottom bend in the rod (point A) needs to be half way along the existing weld (point B) - so the existing weld will need to be ground off completely, without messing up the crimped rim that holds the two halves of the silencer together. Hmm.
Assuming I can get the rod off successfully, what's the best type of welder for putting it back on again?