In my experience the overdrive has usefulness in many situations. I still have to downshift on some hills, and once and a while I've had to downshift into a headwind. With a load I don't get into it much in the highest gear in the main transmission. It can be useful with a load as well, but not very often. It shines, however, when driving unloaded. I don't really use mine to gain top speed, more to drop engine rpm while cruising. Being able to split some of the lower gears is where the aux box really helps getting moving in traffic with a load, although I tend to stay out of the overdrive portion of the box until 3rd in the main with a load.
I'd also leave the available ratios in the rear end functional. Nothing wrong with leaving them in one place for most of your normal driving, but if its there, there will be a situation somewhere where it will be nice to shift into the other, even if its only creeping around a tight parking space or into the back corner of your garage.
