I believe 460 blocks have casting codes like any other Ford motor that should tell you when the motor was asssembled, I just don't know where they are on a 460. After the early 70's 460 use was pretty limited, only some full size cars and trucks received big blocks. By the 80's it was only in trucks so if you can find a casting code that tells you the year of the engine you can narrow down the possible models it could have been in. If you know who did the rebuild and trust his work then $700 for a fresh engine is a steal, but if the builder is a mystery I would pass on it. You could end up with a $700 boat anchor
I've never bought an engine when the seller didn't lie!
Let me narrow that down to running engines. Remember that. Those were expensive le$$ons. I will only buy cores that I have rebuilt so I know what's in it or if they say its running I just assume it needs rebuilt. That way I don't get burned.
Ask the following questions:
What does rebuilt mean?
What old parts are re-used?
What new parts are included in the rebuild?