[LONGHAIR]

This is a bit of a no win situation.
That truck was designed/built in carburetor times and pre-overdrive too. The thing you have to realize (if you are not old enough to remember) is that speed limits were lower back then.
In 1974 the government shoved this down the throat of the states by withholding Federal Tax dollars to the states to force them into it.
The original intent was to reduce fuel consumption. Then the propaganda of "Safety" was used to reinforce the narrative.
The auto manufacturers started working on both. This gave us fuel injection and OD transmissions, which improved mileage and emissions at the same time.
The safety bit gave us anti-lock brakes and airbags. These came along over several years of testing and improvement, becoming essentially standard.

Now we have lane-departure warnings, smart cruise control, etc. Self-driving cars are on the horizon, I imagine with the goal of fully connected cars that cannot crash into each other. (Probably with the ability to actually drive them yourself, but still override your input to avoid crashing) I'm thinking that the ultimate goal is that you would have to enter a destination into the connected car, which helps it even more, since some centralized controller would then know where all of the cars are going.

Sorry, ran off the rails a bit there ^^^
All that to say Big Block, carburetor, gear to pull = lower fuel mileage and reduced highway speeds.
In today's world, you are literally messing with an antique. There are some workaround's, but they are likely to be more costly than valuable.
A smaller, more fuel efficient engine, with a compatible OD and higher gears can help quite a bit. The lower 1st gear of an OD will compensate for the pulling power and the higher final drive ratio will bring the RPM down on the highway at the same time. But, that is effectively a new truck stuffed into an old shell. Do-able, even more-so if you do all of the work yourself, then you end up with a cool old truck in the long run, but at what cost?
Superlow budget.....nope.
Swapping to a 3.73 gear is the economical attempt and probably worth a try, but it's not going to get you all the way there. Even this is a $500 option, doing it yourself, which I generally do not recommend, unless you have done it before.