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Really for the amount of work for an engine swap even a bolt in one that is free/super cheap, it probably isn't worth it for the 3mpg or so gain. But then again the gear vendors unit will get about the same mileage gain and unless you are climbing the rockies daily with a 5 speed it won't be worth it. You could drop one size in the rear end gears and get about the same gain as the GV unit for nowhere near the cost.
The easiest and cheapest things are what I would do first. So if you have a belt driven fan swap to an electric fan. Just find one from a Taurus is what I hear is a really good one to use. That by itself is good for 1-2mpg. Changing to synthetic fluids all through it next time you change them will probably get you another mpg as well as a smoother running engine. Filling in the huge open areas underneath and trying to smooth the underside as much as possible would probably be a big help but I am not sure how much mileage it would get you, probably a mpg or 3. Also if you have a roof rack you don't use take it off for another mpg.
All that stuff costs very little and can get you more mileage than a GV unit that adds weight to the drivetrain. So even though you have a gear split, when not using it you will definitely notice a drop in power. Right now if you can hold 4th going up a hill right on the edge of downshifting you will have to downshift after installing the GV unit due to the extra drag on the drivetrain. If you want a GV unit for other reasons than mileage it might be worthwhile. A 5psd with the GV unit would be a pretty good combination if you are always hauling 10,000lbs and needing to go up hills. With an automatic I am just not sure what it gives you other than a taller overdrive. The torque converter kind of makes the split gears useless if you use an underdrive unit, an overdrive unit can give you a better 4th gear rpm to use on flat ground while empty, but swapping rear end gears to a 2.73 and just using 3rd while loaded would give you even better mileage and the first and second will still use the torque converter to get rolling so they won't be much different for non race type driving.
So maybe give us an idea of what your priorities are and that will help us understand what you are really after. Are you looking for better mileage without giving up your performance? The two are not always mutually exclusive but it will greatly limit what you can do to improve your mileage. Are you just mainly wanting better mileage while still being able to haul and tow things? That means you can give up some power and just go slower while under load. Are you willing to do engine work like a swap to a smaller cam? It would help us to know what you really want that way we can give better advice.
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