The gains are not in the fuel economy, they are in drivability.
With a 3-speed you have a 1st gear that is a bit high. You run a little deeper axle ratio then ideal so you can pull grades in top gear and to help take up the lack of a deep 1st gear. So at any given speed you have 1, maybe 2 gears you can be in.
The NP435 simply adds an extra deep gear in front of the 3-speed gearing.
Now lets look at a newer 5-speed. you can gear a little lower in the axle. The lower gears of the transmission are easier to drive this way. At highway speeds you drive at a lower RPM in OD. But, you have the option to downshift a gear. Need to pass, pull a steep grade with a load, drop out of overdrive into 1:1 and the extra gearing will wind out the motor for that extra power. then back into overdrive. That is where the overdrive shines. Back when I had the NP435 and 4.11s I could cruise the highway, but I only had one choice in gear, top gear. If I lacked power on a grade or wanted to pass someone it had to be done in that single gear or slow down enough that I could drop a gear and really listen to the engine scream. After the 4R70W swap (went to automatic as well as the overdrive) and even lower axle gearing I could cruise at lower RPM and be more comfortable. But when I wanted to pass that semi on a 2-lane road I had a gear I could downshift into in order to get the passing power I wanted.
So there are gains that can be had. fuel economy is minimal, overall driving is experince is a lot more. Depending on the starting point setup as the reference point for fuel economy gains. If you have 4.56 gears and stock height tires then an overdrive will be great. Few of us are that far off as a starting point.