Try harder? I would be surprised if they snapped. You might try hitting them squarely on the top with a hammer lightly several times. I say lightly because if you go to hard it may be possible to crack the head.I was going to say vise grips or double nut but you have those bases covered. I'm presuming you are switching to studs for the rubber valve cover?
Trying harder with no fear of breaking the studs worked out OK...drip-free now with the installed rubber gasket