There are lot's of 3 speed Overdrive trannys out there from '50s GM cars and pickups that would bolt onto your '57 and use the stock linkage. Once engaged with the cable the OD works automatically. With a couple of wires and a button you can also work the solenoid at will to split each gear giving you a 6 speed. Borg Warner ODs are not hard to understand you just need to study them a little. Working reverse with a separate cable could mean disaster if you forgot to take it out if reverse before shifting into low. We get programed to shift into a gear and the linkage takes care of shifting out. One quick back up and a bang into low to get into traffic and you have a no speed!
