You know there's no big secret to getting good mileage, right; you just slow the engine down and lean it out until it won't maintain speed, or burns itself up trying.
For a guy who typically drives 12000 mile per year, the savings from 25mpg to 30 mpg is 80 gallons. What's 80 gallons cost down there? Well, at $3.00/g that would represent an annual savings of $240bucks. Over the life of the engine, say 120,000 miles before freshening, you could save $2400 bucks.
I bet you a dollar,an aluminum block is not gonna save you $2400.
What will save you money is an extremely small final-drive ratio, and a well tuned engine......and driving style.
But you'll need a big enough engine to achieve the power, to get up to and maintain your chosen cruising speed, in your chosen chassis. and you will need enough reserve power,to pass and climb hills.
There's no substitute for cylinder pressure. But too much tends to break things, so, sometimes, a little bigger engine,operating at a little lower pressure is a better idea. And some engines do not take kindly to pressure, at all. Some designs do....