It sounds like you maybe way beyond this point but this is how I did it:
Before I cut out any rusted metla I measured from the body mount up to the brace itself. Then I spaced it up and then I clamped it to the piece of steel that covers the brace. I plug-welded the brace to the cover and then placed the assembly up to the vertical part of the trunk floor and plug-welded it to that. ( 5 plug welds in an X pattern) Then I welded that assembly to the inner wheel house.
Hope this helps!
Ross