My opinion is, aftermarket is purely a crap shoot.
One brand, regardless of price, may fit perfectly on one application (or production run for that particular part), and another may fit horribly on another.
I work in the aftermarket business. Not necessarily vintage Mustang specifically, but we do sell (some sporadic) vintage replacement parts as well as current late model stuff. The only consistency that I've noticed is the inconsistency, and it just isn't in sheetmetal parts (ie. lights, wheels, fender liners, etc.). Unfortunately, that's just the way it is, and most likely is going to be. On the bright side, a vast majority of the parts that go out of our door daily fit and function very, very well. Sometimes, better than OE, as I have seen poor fitting OE sheetmetal right out of the box first hand.
I would absolutely suggest buying a large item such as a '71-73 hood from a local vendor that will allow you to inspect for shipping damage (very common) and fit to your particular car to avoid bad feelings about the purchase. I plan on a fiberglass hood for my car. I fully expect to have a somewhat large amount of time getting the gaps correct and removing waves in the surface. What I don't expect to do is repair heavily damaged corners or fragmented edges from being dropped on the edge of a dock or similar.
I would also expect a sheetmetal hood to have few (or none if you're lucky) of those issues if I were buying one.
Certainly not meaning to confuse things here, but would much rather you be aware of what I've experienced.
