I don't think you are going to be able to turn the engine over with oil in the cylinders and the spark plugs installed. If the engine was filled with oil all these years, I would drain it to see how much really comes out. If it has really been in a dry barn, I would think your probably in pretty good shape. I wouldn't pull the head yet, but do a little digging to make sure there aren't any mice nests in the cylinders from an open valve. If you drain it, and can completely turn it over by hand, your probably in pretty good shape.
