This post has been moved to the Technical section.

If you’ll look closely at the carb on the 239, you’ll find that it’s a two barrel carb.  Doing a general checkout of the 239 and insuring it’s mechanically in good condition by doing a compression test and vacuum check would be at the top of the list.  Then insure that the vacuum advance chamber on the distributor is functioning properly.  If the distributor is not advancing properly, performance will be dismal.  When properly tuned, the 239's actually run quite well.

If all is working properly and you're still looking for a small boost in power, then the four barrel setup from a 256 Mercury would be a bolt on.  Using the Merc 256 intake manifold with a carb adapter along with a later model carb in conjunction with a ’57 and up distributor that has the ’54 distributor gear installed is also an option.  You're only talking 20-25HP with these changes though.