[Bernica]

IMHO, if you find a clean NOS 427 block, you're still gonna have to send it to Keith or Gessford, or someone as reputable on FE's, then you're going to pay for all the corrective machine work to get it right (yes, even on NOS blocks).
They don't do this stuff for free, so you should factor it in. I have seen NOS blocks that were close to trash once the FE guys looked at it.