Into the weeds engine design for fuel efficiency discussion.

As for the engine:
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. The more efficient your engine is, the less fuel it will take to make a given output AND fuel-economy is a good measure of it.
> Power allows less rpm. Less rpm makes fuel-economy.
>but; Very low rpm runs into difficulties with;
1) a late closing intake valve pressurizing the intake, followed by
2) a lack of adequate fuel-atomization with a big carb
3) short power-stroke duration allows energy to be wasted by sending hot hi-energy still expanding combustion gasses straight out the tailpipes
4) cams with long overlap periods, allow headers to pull raw A/F mixture into the exhaust system
5) a lack of Cruise-timing, with factory type timing systems

The problem with running a very high cylinder pressure, on cheap gas, is the inescapable onset of detonation....... which is aggravated by low-rpm.
My experience in this area, is that at 65=1600 rpm (my car), the power-timing wants to be 18/20 degrees at WOT, but 50/56 degrees at cruising. That means I need a vacuum advance can of say 53 less 18= 35 degrees. AFAIK, one does not exist. The best I could get with the modified stocker was 20 degrees; so no matter how you slice it, my engine will be running about 15 degrees retarded at 1600 rpm and cruising. I solved that problem with a dash-mounted dial-back timing module.
The thing about that is, I gotta remember to dial it down when coming off the hiway.
In the end, I found so little fuel-efficiency difference between 1600 and 2200, that I settled on a gearing that gets me 65=2240; for which the cruise timing wants to be 60 degrees, but the factory parts can only be modded to about 40 degrees, add the 15 in the dial back=55* of cruise-timing, and close-enough.
My solution to the detonation problem was running alloy heads, and an AirGap intake with fresh cold-air..
This 360 combo has run cylinder pressure as high as 195psi on 87E10, at a power-timing of 34*, all in by ~3400.

I gotta tell you tho;
The slower you run the rpm at cruise rpm, the more closed the throttle is likely to be. Depending on the cam, there comes a point where reducing the cruise-rpm any more, will start to require more power, and therefore INCREASED throttle opening.
No matter, the point is this; Even tho your engine may boast 11.3Scr (mine), and may crank 195psi at 350rpm with the throttle at WOT;
those numbers, by themselves, are almost meaningless when cruising with the throttle barely open. Because,
if your body/chassis requires say 40 hp to go 65 mph, you will adjust the throttle until 40 hp is being produced. The trick to getting it right, is to select a cruise-rpm that meets the power-requirement with the smallest amount of throttle opening and with the right amount of cruise-timing that the engine actually asks for. After that you just lean out the carb until she loses speed and or it starts requiring more throttle opening again, to get the required fuel to maintain your chosen speed.
IMO any SBM can make enough power at 1600rpm to cruise on, but the bigger it is, the less throttle-opening it will take. And also IMO, the longer the stroke is, the more energy can be extracted out of the fuel, before the exhaust opens.
But I also found out that 65@1600rpm was not at all better than 65@1800, which was only a lil better than 65@2240.... lol.
That 367 combo easily out-performed any 318 ever produced in terms of hiway fuel economy, yet that combo went 12.9@106 in the quarter with; 3.55s, 2.45 street tires, and @3650 with me in it, spinning for a long,long way down the track.
IMO, that combo was strikingly efficient.

So let me rephrase this.
Cylinder pressure is your friend. Pressure makes heat. Heat makes power. Power allows less rpm. Less rpm makes fuel-economy.

It's not about the max pressure by itself.
It's about the whole combo.
Which is "Effective-Pressure", which is what your engine can make at various throttle-positions; and NOT at WOT.
The more pressure that you start with the easier it is to tickle the maximum number, until you hit the detonation wall.

OOPs I guess I shouldda read ALL the comments ..........