As mentioned in the discussion here, the Fairey may not actually be mythical. However it still is pretty much obsolete out of production and unobtainable. If you were to manage to clear those 3 hurdles and have one to install in your vehicle, if anything ever broke on it or wore out, chances are you would be back to searching for unobtainium.
The Fairey man himself has joined the discussion and I'm fairly confident you would not find anybody with more knowledge of or access to these unit.
It really is not a matter of which od unit is better. They are different and serve different needs and purposes.
Considering that by simply swapping in 370 gears and 35 inch tires, you can significantly overmatch the power of your stock F engine for running down the highway, or even moving around town without being sluggish as hell... especially with the three speed transmission... In my opinion you are chasing rare and hard to find boutique pieces which will be not be any cheaper than they are available for no real gain in function.