Black and Decker Heat Gun, then squirt the site with penetrant
Torch flames and aluminum just don't mix if your going to heat it. Flame heat changes the crystal structure of the case aluminum making it brittle. I use a Black and Decker paint removal type heat gun, heating the bolt head and close area site. While hot give the head a good tap with hammer and punch and then quench with penetrating oil and let sit until cold. Lost very few bolt holes or snapped off bolt heads evers since using this method. Even when it breaks free a bit, add more penetrating oil along the way to gently ease it out by both in and further out turns.