Hi Paul
several Brits have installed the Hewland box in the more high performance GT40's including Trevor Tailor in his extremely quick yellow GT40.
The Hewland box is however very expensive and is really only suitable for track use with its straight cut gears.
I also noticed on the original FAV drawings detailing the rear suspension, that reference is made to the Hewland trans, it appears that Ford must have seriously considered using it, for it to be detailed on the drawings.
Regards
Chris.