About that fan thing.......

The "27 HP" it takes to turn the fan is at full rated RPM. It is substantually less at lower RPM's. The marketing of the alternative kinda' left that part out. Also, power requirements increase exponentially with increasing RPM, so at 50% rotational speed, it is much less than 50% of that 27 HP.

The engine driven fan is many times more efficient than any electric setup.

Both fan types use power from the acc. drive belt. The electrics lose efficiency through the alternator, wiring, battery, and motor(s). The engine driven fan loses efficiency only due to its mass and operation period. It is "clutched", but is never really "off".

Sure, it takes more HP to turn it. And it turns when it doesn't need to. There is a better alternative than electric, if you goal is efficiency.

If you were to reduce the mass and capacity of the engine driven fan to near the capacity of an electric, it would be much more efficient, except........when it doesn't need to turn. A thermostatic controlled on/off clutch would also have to be used. Electric, like the A/C clutch, or air operated, like OTR applications would work well. The viscous coupling (fan clutch) will have to go. A remaining drawback is the low engine RPM operation.

Now.......
If an electric fan is capable of keeping a 6.5 cool, a similarly capable engine driven fan would be more efficient. Electrics have generally about a 50-60% capacity of the OEM fan. So, if you were to reduce the capacity of the engine driven fan to near that of the electric, it should be more efficient. The capacity can be reduced by downsizing the mass, and rotational speed. If it doesn't work, then the electric would not have worked under the same conditions.

There are still other factors to consider, like water pump speed, climate, and load, but I think the maximum efficiency can be had w/o an electric fan. Perhaps a combination of both. An electric for lower, and engine driven fan for higher RPM's. The OEM fan was designed to work (or supposed to) while operating at the maximum capacity of the truck, so it has to be overdesigned for less than those conditions. An electric fan alone would be no match under the same conditions.