As you can see, I left a little extra on the top side of the spliced donor section. I did this for two reasons: (1) I can always cut more off... And (2) I figured it would be best to tack the "rear face" of the fender first, and then form it to the match the natural curve down. From there, I slipped the skirts of the donor fender into the stock fender, and traced the line that I needed to cut. Broke the tacks loose, and cut the along the line that I traced. This gave me a 100% match-up.