If you are looking for mileage in a 4bbl upgrade your search should begin and end with a thermoquad. Stock 4bbl intake is fine, or Eddy performer. If it is just about mileage though, if the lean burn is working leave it be. Get rid of the factory exhaust, it is one of the worst (most restrictive) I have seen. New y pipe with 2.5 single run out the back will be more than enough for stock configuration 318.