Pulling the spark plugs and using penetrating oil is the way to start cranking it over by hand.
If that does not work after soaking for a couple of days, you have have to take the heads off and heat the pistons to free them up. Tap on them lightly with a block of wood and hammer.
I would personally take the heads off and look in the cylinders and see whats possible. If there is a lot of rust then you will probably have bore it out and replace the pistons. It is really tough to know what shape it is in if it has been sitting for that long.
You could also add about a gallon of diesel fuel with the plugs out and turn it over (With the Flex Plate). There are also other products that can be used to flush the engine but it would be best for you to look those up or talk with someone knowledgeable on the subject.
If it wont turn over, you will have to tear it down. More than likely, its shot though if it is seized. You will need a new cam, crank & depending on how bad the rings are stuck to the walls may mean block is junk
Salt water
What is the best way to free a seized engine that has never been run? It is a 305 in a 1968 Camaro. The original 350 has a broken starter tang so the 305 is in for now. As far as I know the 305 has never been run and has been sitting for quite awhile in the car but has been kept under a tarp or in a garage.
