Should you decide to remove the studs, chances are, if you try to muscle it out, it will snap off right at the manifold. This can turn a relatively simple job into a much bigger headache.
I would strongly recommend heating the cast iron around the stud until it's a light cherry red. This helps to expand the cast iron, loosening its grip on the stud. Once hot, carefully try removing the stud.
If it doesn't move easily or feels completely seized, douse it generously with a liquid penetrating oil. Kroil is excellent if you have it, or any equivalent quality penetrating oil will work. Let it soak in for a bit. Then, try heating and removing it again. Repeat this entire process (heat, attempt removal, penetrating oil if needed) as many times as required until the stud starts to loosen.
It takes patience, but this method significantly reduces the risk of breaking the stud and saving you from potential drilling and extraction.
Good luck with the repair!
