With a little tinkering and some creative welding and machining, mount a Saginaw 3-speed behind the main trans, facing backwards. High gear will be direct drive, and when it's "downshifted" to second, you've got overdrive. Then all you'll need to do is fabricate an electric or vacuum shift system to shift from 2nd. to 3rd. gear. Leave the linkage off the 1st./reverse shifter, or lock it in the neutral position somehow!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!