i've gotten 25 mpg highway on a long trip as well. at the time i had a taller ratio 2.75:1 this was on my 351 cleveland also.
what murders MPG is the accelerator pump/power valve, that is why a light pedal slow acceleration increases MPG. When you stomp on it 30cc of fluid goes right into the engine instantly. additionally the Power valve will open and dump like 1/2 a gallon into the engine. Plus the cruise jets are running and the transition slot is driping fuel.
so every time you get on the pedal, first your dumping pump shot into the engine from the accelerator pump to compensate for lean condition, the engine vacuum then drops and the power valve kicks on, so 3/8 or 5/8 tube worth of fuel is dumping into the engine for as long as you hold the pedal down. plus the cruise jets are flowing down into the engine. that is why for street driving red light to red light you get like 4-12 mpgs even modern cars can get like 6-9mpg. It was a joke on some cars that you reved the motor and watched the fuel gauge drop 1/8 at a time, and that was actually true on the 1970s jeep cherokee where in offroad configuration you got 1-2 mpg and 4-6 on the highway, the dodge power wagon was just as bad also.
most guys want more power so they also increase the carb CFM and increase the exhaust size, flow more in, flow more out equals more power and drop the MPG even more. that is in addition to what Cubic inch the engine is.
there is more to it also the heads on the engine and intake valve to exhaust valve size ratio has a tremendous effect on MPG. i read somewhere the closer you get to IN/out valve ratio being 1:1 the better the mpg.
this is why in the fuel crises days the Inline 6 was the best option for 71-73, coupled with a single pipe exhaust and 2 barrel carb and the tallest 2.75:1 ratio you could find, and it was possible to get 35MPG in a 71-73 mustang on the highway. and street mpg would be in the 9-12 range, some people would advance the timing and increase throttle rpms and could even get 16mpg on the street with a light foot and learning how to roll the car with a higher idle.
This is why the vacuum advance is so critical and it is the least understood part of the motor for many many engine builders. they all think those are evil and they always disable them and run the engines on mechanical advance only.
basically the vacuum advance advances timing depending on load in addition to the mechanical advance. that means as you accelerate the load is increased the vaccum advance advances timing which raises engine RPMS and gets you to your desired Speed faster meaning your lighter on the pedal without you realizing it. it also helps the motor burn the fuel better by adjusting when spark occurs. so the engine runs more efficiently. just tuning a distributor for the street could bump MPG 10 or more.
so lean out the motors just shy of detonation install a smaller diameter exhaust instead of 3" or 2.5" you go to 2.25" and the increased back pressure and carb double dip from resonance will increase MPG by itself. but the seat of your pants may feel slower.