Hi John, boxing it off is an option, in the past I have de riveted that section, removed the inner strengthening piece and replaced it with another piece of channel and bolted it all together which worked well. I've also seen it boxed with a bigger piece of channel on the outside.
This Idea of reinforcing the chassis around the rear spring idea wasn't one of Toyotas best ideas. I've had a fair few 80's and everyone one is the same.