[Oldsguy]

Pauly, if there is any bolt shaft left at all, remove the manifold if you can to expose the remaining bolt that was broken. Soak that sucker with PB Blaster about six times, one time a day for six days. Each time you soak it, tap the end with a small hammer, just enough to loosen any old rust or debris in the hole or on the bolt shaft, exhaust bolts and hardware rust a lot. On the seventh day, hit it one more time with blaster, dry it well on the shaft and grip it with a pair of vise grips and try to break it loose "righty tighty, lefty loosey". If the bolt shaft itself isn't too damaged or rusted through you have a good chance of loosening it. If it breaks off again flush with the head surface then it's time to take it to the machine shop.