You need more heat...ideally oxy/acet...try MAP gas, a low cost alternative, which you should be able to use with your current propane head. If the top of the studs are protruding above the manifold flange, and are rusty and corroded, try to clean those up as much as possible before you back out the studs. Or, if you can get access to one or both of the studs from above, drill a hole into the stud from the top so it can collapse. Otherwise, if you loosen the stud and try to turn out the corroded upper part of the stud through the threaded hole in the manifold, it will snap the manifold flange because the stud wants to force the hole larger. Ask me how I know!
Tom Mooney