You can take it all the way down to 54 1/2" and get a 6" lip on the wheel if the wheel you want is available that way and have a little wiggle rom to shim it left to right as needed. The Wilwood brake kit is excellent for staying away from the suspension and frame rails, other brake brands can have major interference issues.
Keep in mind some 3 piece wheels have limited back spaces, usually within 3/10". So depending on the wheel your going to run, may depend on the exact width you make the rear. If it is a 2 piece wheel they can weld the center wherever you want it, but I doubt your doing a 2 piece 12" wheels
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Frank Serafine
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