71 slant 6 - air filter mount

The literal answer to your question is: "you can't". The question you meant to ask was "where do I find one of these things?" The answer is, from somebody that has a couple of junk carbs laying around. So the reasonable thing to do would be to post a Wanted ad for a carburetor air filter mounting bracket for a Holley 1bbl (or whatever carb you have).