Usually the Gear Venders units are a 0.8 overdrive, or a 1.25 under... they are either over, or under, but not both. They are shifted electronically, by a button that can mount on your existing shifter. I believe they are rated for duty of 35000 lbs.
Going to a NV4500 will give you a slight loss of your granny gear, but it will give you overdrive in return. The gear splitter overdrive would give you overdrive, and keep your low granny gear. It is also stronger for towing as the NV4500 is weak in 5th gear.
