An overdrive allows normal gearing until cruise speed , necessary if you have a load on board or are towing .

A taller gear ratio will affect the pulling capability of the original engine that you have , especially on hills , even when unloaded.

Your standard diff ratio is 3.9 , if it sounds too busy , you might like to try removing the big fan blade and installing an electric unit as a test for sound comfort.
If it is still very busy , it may be the old engine is getting a bit loose and no amount of gearing will fix that.