Tauro, I was looking to buy a NOS 427 block off of ebay that Keith Craft had started on. The block was NOS, Keith vats the block, starts checking it out and it had three stress cracks! One of the cracks was right across the two cylinder walls on the deck side! A NOS block, how does this happen? I don't know but it does so listen to his advice. It is very tough to purchase something like this from a far and not know what I truly have. Like others have said, the originals had they're own issues and a lot were discarded back then.
I understand the "originality" issue but unless you have a CSX Cobra from the sixties, it would be nice but it doesn't had substantial value to the car. Do you have all the "other" date coded parts that match the same year, etc? Example: a 65 "R" code HIGH Rise engine complete. A complet Holman/ Moody Nascar Tunnel Port engine from Richard Petty's stock car, etc. The engine in a car plays a role to some degree but in the end it doesn't add much to the value of the car, unless it's special or has significant historical value. It"s really the one item you spend all most of your money on and hardly ever get it back.
I think calling a local expert and having them check them out is a good idea, I would feel better knowing what I'm selling and would sleep better too. I would think the NOS blocks these days (68 to 73 service blocks) are really only worth $1500 to $2500. Anything above that, it's just better insurance to start with a new Pond or Genesis block. It just depends on what you have and to feel good about the sale too. Now, if you could prove it came from Holman and Moody or it was in the stack of "original" Shelby frames he had in a warehouse from the sixties, etc that would have more value.
In the end, I think "most" people want what's right for their application. If they have a 65/66/67 then they find that certain block. If not, they settle for the next best thing, then the next and so on. As Keith has stated, if the new offers hadn't come along, then ALL of us would have to accept the NOS price what ever it might be. I'd be interested but to a point because I'm realistic, then it would just make sense to go the new block route. PM me with your info when you find out. My project doesn't have a "NOS" stigma.
It just wants a good starting point. Happy Holidays out there! Matt