3.9s will give more launch than 3.45s but will top out sooner and higher revs at crusing speed... 3.45 are a good ratio i think more launch but without sacrificing cruising speed and fuel economy.
Go 3.45s as foam said every VL auto at the wreckers has them, then go to jaycar and get a speedo corrector kit to correct the speedo and you're laughing.