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.
That is the best method. I have widened fenders of this era. The only thing I would add is once you have the 2" strip cut and rough formed to shape I would run it through an English wheel to give it a modicum of crown for both strength and looks and to minimize the amount of filler needed. Start the tack welds on top near the center and work your way toward each end. I'd build a plywood buck to hold the flange straight - it will save you grief down the road getting them straight and flat again
