What else can I use as a shim for the starter

how thick do we need to go to avoid clashes? Seems to me the Engineers spent a few over-time hours to get it where it's at....do they recommend shims? sometimes they are required...like for differentials...so you can trace the profile on an aluminum can and use a tin snips to craft your own...only trouble is we don't know the thickness that we need...do we? if a trial and error situation would be certain that the starter is the correct model and put it squarely in the hands of your parts supplier....for HE is the one who's sellin' you a part that will work....and not CRACK housings~

Not sure how thick I need to go my buddy's dad works on vehicles an he said its hitting the fly wheel to hard