I suppose it would depend on a few things. Compared to the original 4 speed, most overdrive transmissions have a much higher gear ratio in 1st and 2nd gear, which can affect the final drive torque for getting the load moving. However, the overdives do tend to have better gear spacing, making for easier driving with less rpm drop between gears.
With an overdrive, you would still have a 1:1 final drive gear option, same as the stock 4 speed. That would make no change. Once you're rolling and up to speed, shift into overdrive and you get the lower rpm you're looking for. Changng the final drive gear in the differential would actually cause the final torque output to be reduced in all gears, not just the final gear in the transmission. Using an overdrive is really the best of both worlds, you still have the diff gear for touque, but the advantage of a highway gear for economy and reduced engine wear.
