Doh, as yer wheel arches are away you should have chopped those out as well, grab as much ready shaped steel as possible, you can then chop to fit as required, saves the technical hammer coming out.
Bought an old sierra estate once only to find the back end was away with the wild haggis (it was in Scotland), took about a week with the grinder, a technical set of hammers and the mig welder to sort. I ended up chopping up a couple of old car roofs up to make the required panels and box section reinforcements and then welding them in. Car went on for another 3 years after that until a fiat uno commited vehicle suicide in the back end

written off due to cost of a new hatch door ffs....
By the looks of it yours would be easy enough to fix, bit fiddly but more than do-able. Well worth buying a decent mig welder and learning to use it. Depending on how much you cut off it might be worth while having another fronty handy to measure up so you get the replacement panels in the right place.
Good luck and I hope it's not raining if you do the thing out in the front yard........ mig welder, rain .... electric

This posting may have been helpfull, on the other hand probably not... Either way, I may have had drink when posting it in which case it's probably not as helpfull as it could have been
Work safe, and remember all technical problems can be resolved with a hammer, the trick is knowing where to hit it and how big a hammer to use (I often find it may also help to have a spare bit in the shed to replace the bit you have just mullered

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