Question About Vibration Dampener Torque and Alternator Bolt.

If it's loose with a 5/16-18 bolt, you have a problem.
Yep, there's definitely a problem. I attempted to tighten it up with the belt tensioned and it stripped out the threads long before any reasonable amount of torque was applied. The original alternator was removed around forty years ago so the bolt is long gone and this alternator is an overhauled replacement from Autozone. Just out of curiosity, is there any chance the one they gave me has metric threads? Is that even possible?