Hi mick53 . . .
First things first - make no mistake I am no body-man sheet-metal guy. My fab skills are minimal.
That said - from what is available online and what I have seen in articles the guys who have done this don't use a donor fender. They simply cut a straight line (square to the flange) and then weld in a strip of flat stock (of the desired width) to rejoin the halves.
This approach keeps the basic shape of the fender while widening what is characterized as the flattest part of the contour.
If you have never done it before I would find a sheet metal guy who has.
