The Air/Fuel ratio gauge will be a big help finding out where you are.

Since you desire better fuel economy
you should shoot for a high air/ fuel,
at least 18 at highway cruise part throttle.

If you switch to new iridium tip spark plugs and renew the ignition parts you might be able to successfully ignite 20 to 24 air to fuel.

You should consider swapping to a modern low rolling resistance street tire like the Bridgestone Dueller HL or similar in a tall thin profile. Run the highest air pressure as it improves emergency braking, reduces hydro planing, even though it will wear out the center most tread bar slightly earlier.
This tire change could improve your fuel economy by as much as 2 MPG.

The picture of your truck shows no front air dam below the front bumper.
A large air dam,
coming within 4 inches of the pavement
could improve 70 mph steady MPG by 1.0.
An air dam can be made cheaply from black plastic "lawn edging" with the round pipe like moulding down near pavement.

For further aero improvement consider covering the back part of your cargo box top from the tailgate forward about 4 foot, leaving the 2.5 feet just behind the rear window open to the air.
I know that sounds a bit odd,
but that gives the best aero improvement,
slightly more than a full length tonneau cover.

If the original EGR valve is still on the 400 V8
take it off if you try adjusting the air/ratio up to a high number like 18 to 24.
EGR and lean burn do similar things by reducing engine pumping losses on the intake side but you do not want to do both at the same time.

In an earlier post you mention a vacuum gauge.
If you try high air/fuel ratios
a vacuum gauge is valuable.
At highway cruise in top gear you want low vacuum: 8 to as little as 4 inches Hg
If the vacuum is higher than this,
lean out the A/F more
until the leanest cylinder begins to misfire.

My guess is that your truck with the
leaned out 400 V8,
3.23 diff,
low RR tires,
aero improvements
should be able to get 16 MPG at a steady 70 mph
when the wind is below 5 mph
and the air temperature is 50 to 70 degrees F.

Good luck.