I think that is fixable without stitching in a new section...of course I might be wrong.
The largest visible rust decay is in the seam between the gutter and pillar as evidenced in the first photo. drill out the spot welds of just grind it away flat with the pillar steel and I hope you can just trim the damaged pillar back to the bends and replace with flat there may need to be a few smaller patches and some tricky weld but I think its fixable without a cut and shut from what I can see in those photos. Where are you located?
And to be honest I think you may be in the world of having to repair as Its rare parts to find these days.
have a look here for how its also been done; Post number 95
http://www.turbosport.co.uk/showthread.php?t=442580&page=3&p=2629748&highlight=#post2629748 JP