I have owned '65 Chevelles and 60's Nova, and have done quite a few L79 "clone" motors. As you already know, you can buy all of the
external "add-ons" to make the engine appearance identical to a L79. Unless you can buy a car with all of the pedigree paperwork, such
as protecto-plate, original invoice, owner history, you're going on the seller's word. Even VIN pad stampings can be faked. My Malibu L79
was a perfect copy of the real deal, but I was always honest about what the buyer was getting.
I also have a 409" car, which was not an original 409" car, but you could not tell the difference unless I told you. Therein lies the problems,
anything can be faked and sold as original. Someone will always believe it and pay a premium for it, unfortunately.
Bobalouie