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So maybe give us an idea of what your priorities are and that will help us understand what you are really after.
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I want to enhance the vehicle across the board, for lack of a better term. Fuel economy would be a goal, but making it perform better in general is the ultimate aim. The Gear Vendors system helps with economy constantly, notably when I'm in 3rd gear. The 3-4 shift is rather picky(it refuses to shift until 36mph, but goes back to 3rd at around 30), so I can force it into an overdrive even at the mid 30's. Also, with double the effective gears, I can haul things better by keeping the engine in the powerband. I've looked at the transmission, it would benefit greatly from a gear vendors unit, because the ratios are extremely wide: 3.06, 1.6something, 1.0, .70. That's just begging for a few more gears. Also, while I know the claims are often different than the results, GV claims that you give up an extremely small amount of power. I wouldn't be surprised that anything you lost in the GV unit, you gain back from the fact that you have a shorter driveshaft(less rotating mass, also, see bottom of post about a way to reclaim that)
Not to mention the double overdrive end result, which would bring the RPM at 75mph to a nice 2000ish RPM, which just so happens to be right at the torque peak.
Also, I think 17/25 mpg result is just on the edge of reasonable to ask, with everything I want to throw into it. I read about a car that, with a conversion to FI and a gear vendors unit, went from 11/13mpg, to 14/17.5mpg, and it only had a 3 speed. Assuming I get the same relative gains, that goes to about 17/24mpg for my van.
I'll also list the things I've thrown into it to help.
K&N air filter(whether or not it actually helps economy, I don't ever need to buy a new one again which was reason enough); aftermarket tachometer(I would honestly rate this above a fuel consumption sensor if you are deciding which one to install first); I just switched to synthetic oil, although I was told that my engine is going to leak due to it so I'll keep a close watch and switch back if so; I also use this oil additive I read about called Restore, and it did seem to help(if nothing else, it felt like it got a bit more power and torque back, and sure felt better after a few miles, but I can't exactly do an A-B-A test with it).
I would eventually like the GV unit, an FI kit(Holley seems to make one that looks pretty good and simple enough, and includes a tunable ECU, which is excellent and would allow me to get exactly what I want out of the engine, when I want it(not that I could choose for the basic ECU anyways; 4 barrel mainfold is the only option for my engine).
Also, a question. I've heard of aluminum driveshafts being used to get back some of the power to the wheels that is lost in the drivetrain, useful for sports purposes. Has anybody had any experience with using that for economy as well? I would think that if you're getting more power to the wheels without increasing the engine's power, that you could throttle back slightly and use less fuel while still making the same effective power, no?