Hello! Somebody had the exhaust manifolds off in the early 80s on my '70 GTX. They used header gaskets when they put it back toget5her. I want to remove to exhaust manifolds because the gaskets are leaking. What kind of headaches am I going to occur?

What would be the first step to getting them off? Do I need to heat the bolts? Or do I just spray them down with WD-40? Also, what type of gaskets should I replace them with? Any ideas or information would be appreciated. Thanks!