I decided that the best bet was to cut out the bad metal and replace it, placing new against old to strengthen the chassis. Even though some of the old metal could be flattened much of it was pitted and corroded and I wanted a straight edge to weld too.
Some of the vans internal panels have been beaten through many years of farm work but I didn't see that they needed replacing. I took some time to straighten them where possible but they will end up covered so will not need to be perfect.