3 speed w/overdrive for my flathead
If your flathead is a 1948, or older, I believe some 36 to 41 Lincolns had overdrive. And, if your flathead is 1948, or older, get yourself a 3 speed floor shift ford tranny, then take all the gears from a manual shift 36 to 41 (V-12) Lincoln.
These gears - the cluster gear is 26 tooth - will all fit your ford box and you will have a tranny so strong it will crush a rock crusher.
After 1949, overdive was pretty common.
Know nothing rodder.