Most of the OD transmissions of the 50's were .70. I'm afraid a 3 speed running backwards in second would be too much overdrive. It might turn out to be a real dog behind an automatic. The old BW OD units were really tough. If your are thinking of something like a divorced auxiliary, I think mounting something like a T85, T86 or T89 behind an automatic would work and you could shift into OD electrically. There used to be a feed mill that used a transmission for a speed reducer. They bolted a home made adapter to the front of the transmission and cut the input shaft to accept a Torrington bearing. They then put a pulley on the end of the input shaft and ran a feed mixer with it. It ran for years and may still be running. There are a lot of old Ford OD transmissions around. You might even be able to put a straight shaft through the front part of the box and support it where the input bearing goes. Any way, I think a BW overdrive unit would be the way to reduce rpm by 70%. It might even be possible to build an adapter to totally eliminate the front box and use the OD unit as a divorced unit.