I have looked everywhere and can't find a carb for a 1957 chevy with the I-6, 235, new or rebuilt.

http://carburetorexchange.com/plp/search.php?year=1957&man_id=10&man_model_id=291

If that isn't the right carburetor then they have a "rebuild and return" option. You send them your carb and they send it back refurbished.