http://mustangpartstech.com/SpeedometerGears.html
See if this link works..... Do you know how many teeth are on the drive gear.? 7 or 8? I'd say 19 tooth would be about right, but you may need to check the drive cog in the trans.
Not exactly a falcon, but these had basically the same transmission...
For entertainment purposes mainly.
Transmission Service and Adjustment tutorial, starts at the 34.00 mark.