410Bruce, think of your economy situation while considering how you drive the truck the majority of the time..
The majority of your driving will use up the majority of your daily fuel....Tailor the truck to that task and let the rest happen as it unfolds.
There are things you can do to the truck, that have nothing to do with the engine, to help with fuel economy.
Lowering the trucks ride height will lower the air it distrubs going down the road...but if the majority of your driving is around town, under 45 mph driving then it really does not matter because below about 40-45 aerodynamics is not that important while at the same time aerodynamics above 65 in exponentially important.
Building a front air splitter to direct air around the truck rather than letting it go under will help.
Installing custom panels under the chassis to smooth airflow under the truck will help.
Running thinner tires, at least on the front to decrease wind resistance, will help.
Installing differential gears to best serve your driving needs can help.
I have owned a few different Crown Vic's (1992 being the first) and I installed 3.73:1 gears just for fun...you know, a bit more pep at the light.
I found this helped my in town driving quite a bit, going from 14-16 to 20-22 without chainging any driving habits...read that to mean I like to accelerate when the opportunity knocks.
The 3.73's did lower my highway mileage, but only by a tiny bit because I rarely drove above 70 mph on the highway for more than a few miles, usually setting the cruise control between 65-72 depending on the highway I was on.
The change in gear ratio helped where I drove the car most which was around town, while not impacting the rest of my driving to matter much.
With a truck try your best to stay under 70 mph because at 65 mph the widn resistance begins to really effect economy and it only gets worse the faster you go....especially with a less powerful ingine like the 300-6.
Concerning the engine and economy, you want to build for the most efficient combustion chambers and the highest port velocity you can within the useful rpm range you will operate.
If it is an around town truck that means from basically 1500 rpm - 3800 rpm, with the vast majority of your driving happening in the 1800 rpm - 2400 rpm range.
Finally, acknowledge the fact you are driving a truck....not the most aerodynamic vehicle by a long shot.
If you have old style, multi tube girder looking towing mirrors they grab air.
If you have roof gutters and chrom trim they grab air.
If you have many different elevations on the door glass, trim, sills and elsewhere all that grabs air.
The brick style front end has a huge frontal area and grabs air.
The overall height grabs air.
The height of the chassis off the ground, with all its various nooks and crannies grabs air.
The way the air flows over the back of the cab, or topper, grabs air.
The taller, wider tires grab air.
The larger wheel wells grab air.......The whole friggen thing grab air in some manner so unless you address those issues you will be working toward a mostly moot result.
Even if you minimize all that, within reason mind you, you will likely not get a full sized truck to attain 20+ mpg in normal daily driving.
This means you should set your sights on attainable results and understand that is where things will be.
If you can get your older truck to 16+ mpg, on a regular basis, be satisfied and happy, otherwise just relegate yourself to the enjoyment of the truck with its lesser economy and concentrate on other things.
