Back in the 70's some hot rod mag featured a 71 Monte Carlo that a guy built.
It had a 307 small chevy built for mileage. I remember that the car could get 30mpg or a little more.
I don't remember what carb or trans but it was geared completely for mileage.
I really think that more cubic inches running at a lower rpm can work really well.
I have been around several older cars that got pretty good mileage.
Smaller bore engines tend to get better mileage that larger bore engines.
At least if they are running the same octane fuel.
I had a good friend that had a 1969 Buick Lesbre with a 350 and a Qjet.
It ran very good I don't mean fast but it had very good response.(timing and fuel curves were almost perfect)
It routinely would get 24 mpg and I saw it get 27 once.
I was impressed because my car would only get 21 and that was the best I could get out of it unless I mixed avgas and cranked the timing way up.
I had a Holley carb and had not given up on them as a street carb yet.(I was young)
His buick was stock except for a low restriction exhaust and an ignition hot box and the timing was up higher than stock.
If that could get mid to upper twenty's then your Impala should be able to do 30mph is you keep working at it.
I had a 1970 Olds 98 that would get between 19 and twenty on the highway.
It had a hot box on the ignition system.
It had pretty high compression and I could never get the timing as high as it should have been. I never mixed 100 low lead avgas in it to see what it would do. I never tried water injection either.
I think if you slow the engine down enough then lots of ignition advance will not be as important. I also think that since they are now adding ethanol to the gas that it is less prone to detonation than it was before.
I don't know why that is since they rate it the same octane but that is my general observation.
The newer 94-96 Impala/Caprice had a slightly detuned 5.7 LT1 corvette motor, and many drivers report 30 MPG with no special modifications. My car is several hundred lbs lighter than that, and my wheels are lighter and narrower, etc etc. That is why I think I can beat that. I need Vortec cylinder heads, and probably an RV cam, though
85 Chevrolet. 30 MPG or bust!