Date code won't apply because all big blocks from 1963 forward were stamped with a partial VIN number on the engine itself. Same for the tranny. You could find a two bolt main 390 horse 427 (passenger version), but it will be stamped with a VIN different from your car's VIN.
Personally, I think that the guy selling the car has been watching to many TV auctions. It isn't worth that much in my opinion but then I have a low opinion of factory SS vehicles or numbers matching; having built most of my cars from junk yard parts installed in an old unloved used car.
That said you can install a 1976 454 (used the same block as the 427) and it will have the same date code as your missing engine because GM recycled the date codes every ten years. On a rebuild, deck the block and bore it thirty over then install a quarter inch stroker crank to make a 496 cube motor. Casting numbers may be different, I can not recall when they changed, and I am too lazy to look it up in my Chevrolet by the Numbers book (Alan Colvin 1965-'69 edition).
You could restamp the 454 block by hand with your VIN (used a 3/16" Gothic Normal), and make up a fake date code ( 1/8" Bookman) based upon your car's build date (should be 10 to 18 days before the build date depending upon how far your assembly plant was located from Tonawanda, NY; were your motor was cast, machined, and assembled).
GM bought their dies from three different sources. I think the only one still in business today is George T. Schmidt & Co. (Chicago).
I believe that you wouldn't be the first to fake a numbers matching car to justify an inflated purchase price. Most enthusiast (especially those who think numbers mater) consider this to be fraud, but I have restamped many 396 blocks for customers that blew up their born with motors in their Camaros (I put the same numbers from the old block on the new rebuild). I often wonder if any my old motors were ever sold by Mecum as a numbers matching original.