I had a similar issue on my manifold block, the way I tackled it was to drill a 3-4mm hole in the centre then slam an oversized torz bit into the hole. Hit the torx bit fairly hard to try and 'jar' the bolt, then put a socket it on it and wind it out... it may not work if the material is as soft as you say but it worked for me in the past when all else fails
hence why me and half pint nick named the car 'bolt'...
|