Shimming the starter ??

[nicksta43]

I have a buddy with a two wheel drive 76 blazer. This thing is his only means of transportation. It's completely worn out and rusted but I'm working with him to try and get it at least safe and reliable. His starter is making a terrible noise and seems to be getting worse. I've never had to shim a starter and was wondering how one would go about it? Specificially how do you go about determining the amount of shims you might need? Any help or advice is much appreciated.