Welcome to the forum
Without changing the differential gears, you will need an overdrive trans. The simplest would be a T5 manual trans, but they are not that easy to find. The next manual trans would be the NV3500. That will require a hydraulic clutch conversion, which I happen to make and sell. For an automatic OD trans. I would recommend the 700R4, but properly connecting the TV cable to the I6 carb will be difficult unless someone makes a kit now that I am not aware of.