#1 Check your parts fit on the origional. Then match it or better it. Whatever it takes.
I spent 2 days trying to make an aftermarket fender fit the aftermarket headlight door, and grill, and hood, and door, all at the same time. on a 67 chevelle.
I ended up having to cut the fender into about 3 pieces, and re-shaping it then welding it back together. (was supposed to be the best you can get)