Henri -- Go to Sears and get this set --
It's called a bolt out kit and they have it in various sizes. I got the 4-piece kit to remove stripped allen-head intake manifold bolts on my '87.
Take one of your bolts with you and get the bolt-out that best fits very tightly over the head.
Just tap it over the head with a hammer and turn it with a wrench. Works brilliantly!