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I am a very long time lurker and haven't posted much at all. However, I am also a big fan of full size vans and Dodge vans and vehicles. The four speed Dodge 46RE is a fine four speed automatic transmision that has a lcokup torque converter and overdrive. It is based on the old very reliable and sturdy A904/A727 design so you really don't have too much to worry about in terms of longevity. Making sure the bands are properly adjusted and good fluid is used is about all you need to do. Use the latest Dexron version, VI I think.
With 4.10 gears you will definitely need to use the overdrive on the highway.
The 360 Magnum is about the best of the old pushrod engine technology that Chrysler did. You won't find much in the way of efficiency improvements in that motor.
As has been mentioned above, the engine is already fairly efficient. Simple things like running syinthetic oil (5W-30, or lighter, maybe 0W-30 or 15 or even 10 if your pressure is good enough) to reduce internal drag, making sure your air filters are clean, making sure you have the correct temperature thermostat, etc...
Old full size vans are "dirty" from an aerodynamic perspective, but as has already been mentioned modifications like an air dam are actually more harmful than helpful in everyday driving. A bellypan might help, and some kind of a kamm-back modification, but really something simple like the standard luggage rack put on conversion vans might be enough to break up the airflow off the back of the van and reduce drag.
Your best options are to drive conservatively, reduce the weight of the van, and reduce parasitic losses from fat tires or too-thick oils. The only modification I would do to improve economy would be to increase the rear axle gear ratio to something like 3.9 or 3.5. However, you are driving a big heavy brick and the lower 4.10 gear ratio will actually help the fuel economy in town in stop and go driving.
I once had a 1984 Ford E150 full size conversion van equipped with a 351 high output engine (four barrel carb) and a C6 three speed transmission and a 3.5 rear axle ratio that I tuned to the point where I could get over 20 MPG highway if I drove conservatively. But where is the fun in that? I typically drove 75-80 on the highway and got mid teens in fuel economy. But the engine ran so lean that I melted the top oil control rings on all the pistons and had to get the engine rebuilt. That drivetrain powers my current 1989 Ford E150 that I converted to MAF based sequential fuel injection. It still gets about 13 MPG on the highway no matter how fast I drive, but runs smoother than ever.
My brother has a 1983 Dodge full size short wheelbase van that has a 225 slant six and A904 three speed auto trans that I have tuned to get about 15-17 on the highway consistently, even driving 60+ MPH.
Vans are built for comfort and work, not for max fuel economy. With a full size American gas powered van getting anything above 12 on the highway is a good accomplishment. Aero mods really won't help much unless you do LOTS of highway driving.
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