66 Dart

So do you know if the motor was fine the last time the key was turned off or has it not been used because the motor locked up? In your original post You said that the 26000 miles was due to a long story. If you cant break it loose from the under side then it would be time to pull the head. From there you may find out more but you may need to remove block and start removing the oil pan. Im surprised that its locked up this tight if the plugs were in it at all times. Maybe it blew a head gasket and was left to rust with coolant in a cylinder. But it sounds like your going to find out soon!
Keep us updated.
Are you planning on staying with the 6 cyl?
Rod