6.0L gas RE vs mpg

Just an observation from my past- I can remember driving a 65 Impala with a 283 V8 and a 3.08 rear end knocking down 27 mpg on the highway at 65 mph. I also had various big block chevies that stock gave me about the same mpg at I get now. I think the engineers figured out long ago there was a ceiling when it came to stretching a gallon of gas due to simple physics. However, while the difference in mileage may not have changed much, the longevity of the motors and the enormous increases in torque/horsepower are not even close. I should also mention that this is being done while laden with emission and electrical equipment unheard of just a few years ago. I recall when power steering and brakes were an option.
I know my 2500 wasn't designed to be an unloaded daily commuter to run errands; it's an all business truck with a job to do.