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My 2 cents:
Going larger is more likely to hurt economy than to improve it. Building for low end torque (generally speaking) means both that you're going to be able to put more load on the engine before downshifting, but also that you're going to be running at a lower percent load, all else equal. Lower load = worse economy.
I agree with others, you need to carefully balance engine power with gearing such that you're around 75% loaded at your desired cruising speed. Every engine has a peak BSFC, and there's an ideal power output for peak economy. For this reason there's also an ideal engine output for your needs. Maybe it's a 3L engine at 3,000rpm, and maybe it's a 6L at 1,500rpm. It's really hard to say without having driven your vehicle and knowing whether the engine is over- or under-loaded, and whether your target RPM is going to be below peak efficiency and start to hurt economy.
One thing you can say for sure is that, all else equal, a larger engine has more parasitic losses. So, going bigger is only a good idea if everything else lines up perfectly.
Adding a second overdrive to reduce RPM will also increase parasitic losses. I'd probably start by trying to cut those, rather than add to them. Get roller cams, minimize brake drag, synthetic fluids, reduce accessory load, do your aerodynamic optimizations and weight reductions. Maybe after that your existing engine isn't loaded enough, so decrease the final drive to reduce RPM.
Another alternative if you're feeling really adventurous is to de-stroke the motor, which will both move peak BSFC up the rev range *and* cut down on friction losses, since piston speed will be lower. Stroking it combined with a taller rear end may also have the desired effect.
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