I'm sure it has a door tag and a vin tag, which would reveal the engine code as a "K".... like 6F07Kxxxxxx. And all Factory-installed 289 High Performance engines had the vehicle serial number stamped into the righthand side of the block near the battery's negative-cable attachment point. The myth that they had screw in freeze plugs is just that. That's a Boss engine thing.
If it's a K Code.... that would be a candidate for a factory resto. Either way, a fun find!
Cheers
Dave