A 1971 Plymouth Roadrunner rear licence plate bracket is spring hinged to allow access to the gas filler benind the plate.
Take the same bracket, and mount it in the front of the GT40, and use a long "choke" cable to activae the movement of the swing of the plate. It will require trimming of the spring so the pressure against the spring does not flop the plate down.
Not sure how readily avaiable the old Roadrunner brackets are, but I am sure other manufacturers have similar ones available that would work as well.
