I had no problem w/bolts on 91 Regal but that car was garaged and immaculately kept. For tuff bolts I :
try to wire brush any exposed threads sticking out on back side. Drill w/brush, or hand brush.
use pointed dentist thingy to clean torx or allen hole
put torx on extension and rap it in good in the first place
plus 3 or 4 more good raps to show iron you mean business
actually it shocks the rust and grip somewhat just like a impact only not as good. (This works great for those pesky phillips head body trim screws)
I try things quick if I dont like the feel I go for torch immeadiatly, small propane torch back before I got my acetalene. Just takes longer.
heat the area the bolt threads into not the bolt. You want to expand the housing or nut not the bolt
You also dont want the bolt soft hot that will ruin the torx or allen socket your trying to hold
Turn a grade 5 or 8 bolt blue and youve reduced it to nearly one of those grade 2 wanna be bolts. Turn it red and its soft. You will round out the head of one of these softened bolts lickity split.
I also never heat when tool is in place. You dont want to soften and remove temper from tool or expand the fit of the tool (can of cold water to keep tool cool isnt a bad idea) sure everything will be hot by the time your done but you mainly want to expand the item the female threads are on.
You also need to hold center of ratchet or extension with other hand so you dont twist crooked on socket,torx or allen. It makes me nuts when I see people do that. Your just looking to mess up the head of said bolt or break socket or snap torx or allen. One hand on each end of ratchet or breaker bar.