To mount a york to a 258 you'll need the brackets off an 78-80 AMC car like Concord, Gremlin, Pacer or any of those very ugly beasts.
If you pull it off at a pick and pull will make it easiest for you because you need the mounting bracket, alternator bracket, adjusting idler pulley and the flat idler pulleys and all the different bolts that are a little longer than those on the block. This will mount the york upright with the alternator right below it.
Also if your york has the input and outputs that are real high, it might not clear your hood in which case you will need the ends off and AMC york which will hopefully fit. I've seen yorks with the input/output ends that either bolt onto the compressor or have are large threaded 1/2'.
Tim has some great pics on his site.
I have to say of all the mods I've done this is the coolest. I love to tell people about my york and how I can run air tools and air up tires OFF MY ENGINE!
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