Ran across a guy on YouTube who found a 1970 Z28 CTB shortblock with the VIN 576323. Didn't know if someone had the car it goes with. V0613CTB Date F100.
https://www.youtube.com/watch?v=BNcesQoR0gA&t=555s
Cross posted on NastyZ web site.
I don't know or have ever met with the owner, but by the youtube it seems pretty legit. He was not looking to sell it, but is only interested in building an early 70's style SBC so if the money is right he may give it up.