Broken Bolt in Water Pump

It sounds like the shank of the bolt is siezed up in the pump. i would try like what crackedback said and weld up a nut to the end and apply some heat to the pump to try and beak it free. if that doesn't do it try and find a spot to put a pry bar so you can hit the pump foward after again applying heat and spray something like freeze off on the bolt.