Unless he is willing to offer a refund if the block or head is cracked, because these are simply unknowns, I would offer him $100-$150 just to open it up for discussion with him and see if he is in the same ballpark.
A wise old man once told me "Everything was running before it was parked - well excuse me - then why was it parked? If you can't hear it run it's just a core".
Even rare items are just scrap if they cannot be rebuilt - or worse if they require co$tly crack repairs before rebuilding can even commence. Which to CNC's point represent unknown risks to you as the buyer.
I am also curious to hear more about this Stude project . . . not mention the existing hot rod with the 356 Packard. Pics please.
regards,
stock49