18 seems like a reasonable expectation.
My grandfather's old Dodge went from 11-mpg,to 16 mpg with 4-speed OD and taller rear axle.
An aero nose/grille-block/ fan delete/Plex headlight covers/front wheel gap closures/side mirror delete and addition of small car mirrors/full rear skirts,and aeroshell got it to 21.5 mpg.On a weak slant-six.It already had full wheel discs.
No doubt the Ford has low 20s potential,but I think you'll have to take it further to get it.
Let us know!
