If the camshaft is not sliding out easily, the center cam bearing is likely the culprit. There’s a good chance that the center grooved camshaft journal has worn the cam bearing such that the camshaft will no longer slide. With the engine upside down, you’ll need to force the camshaft forward by using a long screw driver or similar pry bar placed against a camshaft lobe and one of the interior main bulk heads. Once you get the center cam journal freed from the center cam bearing, the cam should move forward reasonably easy.