Experience from elsewhere, but it may be relevant here:
When I used to paint and repair bikes (did that briefly for others), I always found that the stock panels fit
much, much better than the aftermarket stuff. I just sort of assumed that the same would hold true to aftermarket car parts also. I would take a used bike panel over a new aftermarket panel any day. My same logic goes to car parts too. Now just look out for any breaks or cracks and you are good to go. Also, even with new parts, you are going to find slight surface imperfections, so I am always assuming that I will do some slight body work to get the parts to the way I want them.
BTW, I am selling two wings in this forum, heheh, but I would have told you the same thing that I did regardless