I would start by cutting the inside edge off the existing wheelwell. Cut it along the trunk floor mating area, and then along the top of the curved wheelwell. You should then have a half moon shaped piece you removed.
Once you've got that out you can cut in towards the frameon each end, and then along the edge of the frame. After that the first piece you cut out can be located and tacked in at the new inside edge. Then a strip that is wide enough to span the gap can be made up and tacked to the inner wheelwell, and the relocated inside piece.
Then it's just a series of tack welds until it's all solid welds, and seam seal the welds and paint. I can usually do a mini tub in a few hours total.
Here's a tutorial that will help. It mentions drilling out all the spot welds, but I never do that as it just isn't necessary if you cut along the floor instead.