Not a big fan of heating cast to remove bolts, it almost never works out for me.
Best way I have found is to very carefully center punch and drill it in stages until you can re-tap the threads, or get lucky and spin the rest out.
Extractors are okay if used conservatively, but I've made things worse using them, especially when the bolt is thin and the extractor actually expands and locks it in place on rusty, pitted threads.
I've found that the square extractors work best, as they actually grab in 4 areas, rather than maybe 1, with spiral extractors.
One of my favorite ways of trying not to break a rusty bolt, is to try to TIGHTEN it first, then hit it with penetrating oil and try to crack it loose, then re-tighten again and loosen it again.
Work it back and forth like that, while adding penetrating oil, to try to clean the rust/debris from the threads.
This procedure doesn't always work, but more often than not it gets positive results.
The theory behind this is that if you tighten a bolt, it stretches the bolt ever so slightly, causing it's diameter to shrink slightly, which takes away the "locking effect" and lets the bolt move.
The problem with heat, is you can get too much of it, then when cast cools it actually contracts enough that it LOCKS the bolt in, then you're through.
At that point drilling and tapping almost becomes the only fix.
Mark.
