The door shells have been improved since the original release of the product but I know of noone that has tried them. NPD claims they had resto shops try them and they were very happy.
I used repop doors on my '69 and they fit really nice. I only has one issue with the top of the door on the front edge. The gap between the door and the front fender was uneven. My bodyman fixed it with little problem and now they fit perfect.
I just had one installed a few weeks ago. It is a Dynacorn door that I got from Mustangs Unlimited. I don't know where NPD gets their doors from. The body shop guy asked me if I was sure it was the right door. He had to take a total of 10 mm off the edges and re-roll it in order to get the door to close. It was eventually made to fit but at an exorbitant cost.
I purchased a door from Dynacorn a couple of years ago. My original door was rust free, but had been damaged years before and was full of drill holes from a slide hammer. The door skin on the Dynacorn door was a bit wavey and the door skin would have had to have been replaced to get the door to fit right. The body shop said it would need to much work to get right, so we just re-skinned the original door which turned out great. I think the main problem is the way the door skin is attached to the door.
I bought a couple doors from NPD a few weeks ago. The new and improved ones. They fit great, they have the grain on the inside panel too of course. The only negative is that the area where the decorative crease runs from front to back will require some filler and sanding to fix the waviness. In fact my painter wants to skim the whole door to make it perfect. Not a problem though considering how bad the fit could have been.
NPD's rear quarter fits well too. My painter is still messing with the front fenders (haven't fit the extensions and buckets yet) so I'll hold off on my opinion of the front fenders until the facts are in.
i hear the 69-70 repos fit the best, 67-68 are next, and 65-66 are last. this info came from the head guy for kentucky mustang. they restore a lot of mustangs.