I'm a third generation car guy.  When I say car guy - that's three generations involved in building Detroit cars. I've been in virtually every auto plant in North America, including a bunch that no longer exist.

Ford was famous for a constant stream of changes; it wasn't uncommon to have multiple change orders to update or improve parts more than once during the production year.  On top of that, Ford ran HUGE numbers of parts at high production speeds - then let the parts "sit" partially completed for months on end.  If there was a defect in manufacture, it often wasn't quickly caught. this makes it hard for  restorers sometimes, as a cataloged part may or may not be the "right" one for a specific vehicle, or that NOS part may be part of a defective run that went out to distrubution before the problem was caught.  Nowadays, most every component is tracked for liability reasons, but that wasn't the case 60+ years ago.     

As for replacement parts - many, if not most, suppliers are furnishing offshore (as in not from traditional Detroit Suppliers) components.  In many instances, the part looks good, but the base metallurgy isn't to a Ford standard.  Ford's reputation was forged (pun intended) on metallurgy - the use of Vanadium Steel in the Model T afforded a light and flexible and durable vehicle, for example.  

When it comes to buying quality Ford parts, there are two suppliers I trust.  The Early Ford Store based in San Dimas CA, has a reputation for providing components which meet or exceed OEM standards.  My favorite, however, is Dennis Carpenter out of Charlotte, NC.  Unlike other parts sppliers, Dennis has taken the time to build his own production facility, making parts to a Ford stadard, often from the same tooling used to produce the part originally.  Dennis bought out my father's production equipment when he retired - the trim clips used on 50s'/60's Fords are made on some of my dad's machines, for example.  FWIW, Dennis Carpenter is the only source I know of for metal Y block head gaskets.  

Both of these suppliers are more expensive - and there's a reason for that.  Is it worth it?  Absolutely.