have you stripped the allen head of the bolt? I would pull the pump off with the venturi still attached and get it on the bench to heat it up and try getting it out, if you have it stripped or broke the head off then I suppose you could try drilling it out but I hate drilling stainless, usually if I have a seized or broken stainless bolt I use welding rod used for extraction, I can't remember the name of it but I will try to get the name of it, it is a arc welding rod that you can use to build up the material on the end of the bolt or stud to grab onto it with a pair of vide grips, because of the heat from welding the fastener usually spins right out, I use it on broken manifold bolts on 720 and 657 engines and rear pipe mount bolts on 947 engines.