Looking for overdrive trani advice for /6, please help

My car is partialy a long-term Science project aimed at getting good mpg. I have a 74 Duster with a 225, automatic transmission. My plan is to have the engine re-done. At the same time I would like to consider swapping out the transmission for something with overdrive (for better mileage) and I still want an automatic. Can someone tell me what the best/easiest swap would be? I don't want to buy new so I'm hoping there is something out there from the late 70's or early 80s that might work well. I did try to search this out on FABO but wasn't completely successful. Thanks so much!!