I had experienced in the past the fields in the alternator reverse polarity.
When that happened it did snap the fusible link.

Run some wire from the battery stud back of the alternator to the battery stud at starter relay WITH A FUSIBLE LINK!
If the link snaps right away,the alternator is no good.
If everything lights up and runs fine,the ammeter gauge is no good.
You can leave this jumper wire in permanently since it takes a load off the ammeter gauge and completes the curcuit between the + and - side of the charging system.
Hope this helps.