If there's any bolt sticking out of the shock/trailing arm then grind it flat, centre punch it then drill it with a 6mm drill 10-15mm deep. Heat it up then get a good quality easy-out into it.
What I tend to do before tackling a corroded or seized bolt is hit it hard a few times down it's axis with a copper mallet then apply some torque to it as if to tighten it. If that doesn't free it up (assuming it's not stuck because the head of the bolt is rusted to the mating part) then I heat it up gradually whilst hitting it with the copper mallet. Tends to work on most things.
