Walter,
Over the years, 1929-1931, the height of the front, (highest part of the front seat upholstery), measured from floor board to top of upholstery, varied from approximately a little over 10" to a little over 14", as indicated in the 1955, 5-1/2" x 8-1/2", "Ford Model A Restorarion Manual, A-1". (A copy of which can be bought from vendors).
However, if I were you, I would make an adjustable wood seat first; second, try different heights, & lastly, make it such that the height is where you or any other drivers feel the most comfortable.