318 MAX fuel economy builds?

Cruise power requirement in an FMJ car is only around 40/45hp.
At WOT, that would be about 21.25 pounds of fuel per hour. But cruising at 2000/2200rpm, I'm guessing maybe half of that, so say 11pounds per hour; rounds to 1.83 gallons. At 60mph, that maths to 32.8mpg . That could be your target.

Effective cylinder pressure would be your best friend; it makes torque; but it will take a lotta miles to recoup the buy-in.
Easier and cheaper by far, is to dial in the cruise timing.
and a 318LA with a 2.45 rear gear @ 65=2000rpm, should be barely on the MainJets to make that....... so any old carb will get you there. The smaller the primaries the better, but the higher the vacuum at 2000, is even better.
If I was planning this out, I would start with a closed chamber /tight squish design, and way more than factory Cylinder Pressure. For cruising on, you can run a ton of cylinder pressure.... you just have to figure out how to get to that cruising speed without detonating the poor girl to death. I vote variable-flo water/meth injection. Ignition timing at cruise rpm will be the key.
Sure this will be pricey, but you want bragging rights so who cares ....... right? Lol.

OK but, since we're talking theoretical, I would start with a smaller bore or more stroke, or both........ and maybe long-runner intake tuned for cruise-rpm. I think a Magnum would be a logical choice ..... with an A500 and a rear gear between 3.55 to 3.91, for a cruise rpm of 2000 to 2200. And if a Magnum is on the table, Ima liking a Magnumized 273, with custom pistons.
Or maybe 3.65 x 3.58= 300 cubes, and 65=1660 with 2.94s, and that A500. Yeah I like it!
Wait a sec, I think I still have a lil 273 kicking around somewhere........ hmmmmmmmmmmmmmmmm

And then; there's a certain super high-compression Alloy headed 367, with Plasma-Moly rings, and a manual trans with overdrive, that runs 65mph just off the transfer slots, @ 65=1600rpm, hmmmmmmmmmmmmmmmmmmmmm, that's a small bore/long stroke 1.18ratio, same as the 318, but with a lil more inherent torque to get that tank-Dippy/FA rolling, and to give it some passing-power.