It sounds liek when it was put back together, if it was ever taken apart, and the bolt was probably stripped when it was put back on. I know on my rear of my sedan I had a problem w/ one of the rear bolts and I have to remove the entire arm and all because of it. In your case you nedd the arm to go back on w/ the new shaft so I'm not sure what to do but to maybe take a old socket and have it tach welded to the bolt and try to get it aloose OR possibly go the air tool route and see if that will break it loose..
Hope this might help and good luck w/ it...
