Slant 6 MPG Upgrades?

On the fan, I'd just put a clutched, thermal mechanical fan on /and call it good. More efficient than a solid drive fan, but the electric fan takes more electric power to run which is more load in the charging system. If the alternator has to work harder it in turn takes more power to run as well.
Argonne National Laboratory did a few studies on engine lubrication and efficiency, which is fascinating, but they also provide valubale data on all of the various power losses in operating a vehicle.
2006 2014
They're a rabbit hole in and of themselves, but the highlight is that they estimate .5% each of the total energy from the fuel is expended on the alternator and the cooling fan. So yes, more load on the alternator does equate to more energy being spent, but at the end of the day you save more energy by switching away from a belt driven fan. Alternators are fairly efficient, not as efficient as the power supply on your computer, but not as inefficient as running a fan off the crank while driving at 60 mph, inducing extra drag.

Now, is it worth it? Not necessarily. The main benefit to electric fans is better cooling at low speeds and idle. A clutched fan is better than direct drive in that you're not fighting wind resistance at high speed, which increases parasitic loss.

Just to be the devils advocate here, I see everyone discussing directly modifying the /6 (with good reason) but as I am sure you are familiar with the "Feather" duster and similar a bodies of the mid-70s, much of the gained economy came from weight reduction. I see that you are already on the way with the lighter weight engine components but the same way you can cut drag times to make your car faster at the strip with more power, you can have the same effect with less weight. That applies to mpg as well, the less weight you have to move around, the less power it takes to move it. Have you considered any type of fiberglass components? At bare minimum bumpers and maybe even fenders, hood, trunk lid?
Just a thought...
I've thought about it some, but at this point I'm not sure how worthwhile that is. I'm trying to strike a balance between removing unnecessary weight and comfort. A decades old underdash A/C system is just heavy and inefficient and barely functioning, getting rid of it doesn't hurt me at all. If I swapped any single component, it would be the dashboard and out of fiberglass/carbon fiber and then painted to match. It's unnecessary weight being made of steel.

How much do you want to keep running a slant-6? The 225 is inherently inefficient by design with the super-long stroke and heavy rotating assembly. If I was to ever intentionally own a slant it would be the 170 variant. I think that would be the best bang-for-buck move in search for MPG and keeping the car 6-cylinder. Could just swap over the upgrade parts you already have. The slant wasn't intended to go over 170 cubes, when Chrysler management asked to make a bigger version the engineers wanted to increase the bore but the Valiant designers were fixated on the length dimension of the engine bay so the only option was to increase the stroke.

IMO you'd be way ahead swapping in a stock 5.2L Magnum V8 with the factory multi-port EFI. Only potential issues would be needing a different intake manifold to clear the hood. Mounting options aren't tough to come by, either conversion mounts with current K-member or find a V8 one. You could use factory 273/318 A-body exhaust manifolds which nobody wants and are dirt cheap and could easily find a V8 904 transmission to bolt in place of your current one. Even with the current gearing you would easily surpass 20 MPG average in such a small and light car and you'd easily double the HP as a bonus. I had a 1988 Chrysler Fifth Avenue that averaged 20 MPG and that is a much heavier car with a roller-cam 318 and 904 trans with lower 1st and 2nd gear ratios. My average MPG didn't even change after I swapped to 3.07 gearing (they never came with overdrive), 4-bbl carb and intake and 2 1/4" dual exhaust off the factory manifolds as well as ditching the spark-control computer for conventional distributor and used MSD box I got for free from a friend.
I like the /6 at the end of the day. It's a fun engine, and its the heart and soul of my Dart. I started with a 170, but when I went to rebuild, a previous owner had bored it way too far over and it just wasn't going to be feasible to rebuild without expensive testing to make sure the block was fully intact. It was cheap and easy to get a 225 that had never been touched and bore it and modify. If I could have stuck with my 170 I would have.