Andrew...Thomas from AZGP gave the following info..
Ok, looked at my car with a friend and my friend told me a story about one day he miss mounted his alt, didn't realize it until he couldn't get the belt on... it was a 3800. Yep same motor. 1989 La sabre.
Check the mounting of the parts.
Actually, one day I miss mounted my PSpump. Doh..!!!!
I'm sure that either your water tube is wrong or the alt is mounted wrong..