Looking at your sketch the structure looks a bit delicate for the heavy removal methods, especially if the bolt is bent, time spent in preparation is not wasted, as if you wreck the tube its engine out anyway.
Suggest removing the engine and carefully drilling the centre out the bolt going up 1/16 at a time as previously recomended, heat is a good trick, use a heat gun for a long time to give a good soak, and a sharp tap should do it, if the bolt is bent then don't use too much force as you may crack the tube at the hole area, if you have access then you could use a series of expanding reamers to ensure that you remove the bolt shank material without damaging the tube hole, as always with these tricky jobs, take your time, if it takes all day to get it out and you haven't caused further damage then its a good job,
good luck,
om15