Take a sheet of paper large enough to trace the wheel opening and tape it to the old fender. Then trace the wheelwell opening along the edge and along the back edge of the fender. after you have done that, mark your holes on the paper. Then tape the piece of paper on the new fender and line up your marks and mark your holes in the new fender. Then drill starting with a very tiny drill and go up until the hole is the correct size. I bought some templates from a Mustang Parts Store once apon a time, it has all the mounting holes on the whole car. I went and looked but it doesn't say the name of the store. Knowing my ordering habits it was from
www.texasmustang.com or
www.laurelmountainmustang.com.