One theory about low RPM and attendant low manifold vacuum is that pumping losses due to carburetor butterfly valve restriction are minimized. That could be negated by richer mixture at low vacuum. But the mixture can be adjusted to compensate.
Most old gas truck engines that I drove when under load ran at about 0" vacuum nearly all the time. The GMC 270 and the 302 didn't even have vacuum advance on the dizzy. That said they also got terrible gas mileage but we're talking a 270 pulling 26,000 lb loaded dump truck and the 302 was a hay shaker. They weren't fast either. An exception I can think of was a '52 Ford F8 tractor with a 332 Lincoln V8. It was set up to run about 2400. pulling a load of hay at 46,000, not exactly a Hot Rod Lincoln but it did have a 4-barrel carb and dual exhausts. It got slightly better mileage than the 302GMC and was slightly faster. On the Ford, the boss wanted us to hold 4"Hg. That was hard to do; one eye on the vacuum gauge and one on the tach, third eye on the road. We also had an I-H truck and trailer rig powered by a 1090 Hall-Scott I-6. It pulled 72,000 and had 2 updraft carburetors, dual ignition, and overhead camshafts. It had lots of power and got about 2 MPG pulling hay over the mountains. It was also set up to pull about 1800-2100RPM. I don't remember if it had a vacuum gauge but the carbs set up quite a howl at 2100 RPM full throttle. It was enough faster than the other trucks that we could unload the same day and make a trip a day.
Engineers also factored in specific fuel consumption to their designs. Specific fuel consumption is pounds-of-fuel-per-horsepower-per-hour. That minimum occurs just below peak torque. So the least fuel consumption would occur at 0" vacuum at about 1800 RPM on these engines rated at peak torque at 2,000 RPM. So the carburetor would be set up for maximum torque at 0" Hg. A
Camshaft design also plays into this equation. Most 235 cams provide peak torque @ 2000 RPM, 261 cams @2200. Fuel consumption on my 3800 with 235 dropped a lot when I went to a cam with peak torque at ~2600 RPM. That is because for highway usage I was running 2800-3000 RPM, WAY above peak torque and at least 1,000 RPM above minimum specific fuel consumption. I made no other changes and went from about 9MPG to about 13MPG. Then came ethanol laced gas and I was back to about 10MPG.
When I put in the 261 the compression bumped up from 8:1 to 9:1 and the displacement up by 11% to 261 and I re-geared it from 5.13:1 down to 4.57:!. Same cam. It now cruises comfortably at 62 to 65 MPH as compared to 58MPH and the mileage is about 10MPG up hill and down dale, even into the wind. But with a tail wind it'll jump to 12. It now gets about 12 empty, i.e. no camper. I think mileage would be better with a 4.88 gear rather than 4.57:1. That would put RPM at 2800 rather than 2600. I notice that the fire tends to go out below 2500.
