The "flex pipe" part of the pipe has a leak and I need to replace it. I purchased the pipe and gaskets and after raising the car on stands I find that the three studs /nuts on the catalytic converter pipe are really rusted up. I applied liquid wrench and let it set, but they wont budge with a ratchet. Before I go any further using an impact gun, what can I expect ? will I have to remove the manifold and replace the studs in the cat flange ? Advice would be appreciated.