Removing stock exhaust manifolds
It helps to heat the bolts/nuts with a torch (propane and propane accessories is ok).
Lots-o-penetrating oil...
Then turn them to tighten them at first to break them loose.
After you feel it break loose, then reverse and loosen.
I find that heat is the most beneficial on old rusty nuts and bolts...
Remember the tip of the cone in the flame is the hottest point (the inner one)....