Might want to go so far as to call a local tool-machine shop for a welding reference. Those guys weld to micron level tolerance (laser weld if required), not that you need this but they can do it. These guys aren't cheap (~150-200 per hour) but if you want the best that's where to go.
As hard as this fender was to come by, I would pay $300-$500 to have done. Call me crazy!