If the PO was right and it just had 300 miles on rebuild . It is seized tight. Keep on soaking and stay after it. One trick used on old seized tractor engines is to fill the cylinder up with Diesel . Let it soak a few days. Put a rag in the cylinder for a wick and light it. The heat produced by the burning diesel will make the cylinder walls expand . Then when it cools down braking any rust loose . But a V8 inside a engine bay that would not be a good thing. So try a nonflammable heat source .