Something to think about with a overdrive is the modifications to the floor/transmission tunnel, mount, and driveshaft you will need to do to install a O/D in your car. I wouldn't mind one in my 70 Swinger but I don't want to beat and chop to make it happen. Mine with a 318 nad a 904 works fine and get's almost 20 mpg highway mileage. A lock-up 904 from a late 80's M body may help with the mileage by a mile or two per gallon. Just my take on it.
