That overdrive unit may not be much help
I have a 1979 F350 4x4 Ford truck which has a C6 transmission and I rebuilt a 460 cid motor for it. All is well and good except I could use some improvement on gas mileage. My question is: Are there transmissions with overdrives that I could use to replace the C6?
You can replace your C6 automatic with a more modern overdrive automatic. Most people who do this swap use a 4speed automatic overdrive called an AOD. Earlier units that don’t require computer controls are plentiful, strong, and affordable.
Even so, I’m not sure that an AOD or other overdrive transmission will measurably improve your fuel mileage, in part because I don’t know how you drive your truck.
In general, an overdrive transmission will yield greater fuel economy in vehicles that are frequently driven at highway speeds and if you do most of your driving around town you won’t realize the benefits that an overdrive can provide.