I wouldn't count on anything coming loose.Give it your best effort,no matter how long it takes.I would give each bolt a good wack with a hammer.Next I would cut the head of each bolt and remove the manifold.Use penetrating oil on the thread/head area and let it soak overnight.Heat the stud red hot and also the area around the stud at the head.Take a water soaked rag and cool the stud.This causes the stud to retract and brakes the bond with the rust.Use 2 vice grips on the stud and try a back and forth motion to get it started.
Good luck.