Yes, you'll need heat, and lots of it. I use an acetyline torch with a rosebud tip. Heat the manifold ears, not the stud, until it gets red, and carefully work the stud loose with a pair of vise grips. You'll probably get half a turn or so at a time, and the manifold will have to be re-heated. Go slow, and don't get impatient. If there's anything left of the broken-off studs above the surface, do the heat and vise grip trick on them also. Drill only as a last resort, and be sure to clamp the manifold into a drill press so you can hit the stud dead center with the drill bit. The least bit off-center, and you'll leave a piece of steel bolt in the cast iron manifold, and then you'll break the tap off trying to recut the threads.
If you try to use a reverse-spiral "Ezy-out" thread remover, you'll just expand the thread into the manifold and lock it in so tight it will never come out. Step-drill the stud until you get to the minor diameter of the thread, (1/8" to start, 5/16" as the tap drill size, in 1/16" steps) then cut the remains out with a 3/8"-16 tap. Slow and careful, and use lots of Rapid-Tap or some other type of cutting fluid, both while drilling, and threading.
Jerry
