[27litres]

Mikki Jayne has done this, and even given the fan two speeds by utilising the aux fan resister circuit.

I'm thinking of following her example, maybe slightly modified...

I have experience building a custom thermofan setup in my old Holden 5.0 V8 Berlina. In the case of that car, I did get much more responsiveness and freer reving of the engine. Ultimately engine power was unlikely to be effected, but the lower load on the engine during initial acceleration made the car feel much faster (and it probably was, slightly). It also gave better fuel economy by about 0.5-1l/100km (2-3ish mpg) on the highway cycle. It sounded better too with that constant fan roar missing!

Yes I know clutch fans do a good job at reducing engine load during 'cool' cycle with their slippage ability, but they still create drag.
Also remember that the drag on the engine to drive the mechanical fan is 35-40% efficient.
The Alternator is far more efficient than that at producing electricity (90% or so) and an electric motor has similar efficiency at producing electro motive force.
That brings the overall electric fan drive efficiency to around 81%, which poos all over the engines mechanical efficiency.
So the resultant EMF drag on the alternator in driving the fan would be at least half that of driving the fan mechanically.

Why do you think all manufacturers now use electric fans (negating packaging requirements in transverse engine applications of course)?
I remember hearing similar arguments during the switch over to fuel injection from carburettors. What a load of crock that was proved to be!
Cars are also switching to electric power steering pumps, electric airconditioning compressors and even electric turbo and superchargers in the chase for higher efficiencies...