I have a 4107 and when I removed them in may I had the same problems. On the
exhaust manifold I just used a little leverage to get it loose. On the
head I have a large crowbar and picked a point to again
lift using leverage. It is just stubborn but remove the exhaust manifold first and be double sure everything else is disconnected. Exhaust manifold is cast so could be cracked easily. Head is heavy and robust.