How do I ID a 4 speed, overdrive or not?

Hey guys,
I recently got a 72 Duster with a 318, 4 speed. It was originally an auto, /6 car. How do I determine what kind of tranny is in the car, did they come in regular and overdrive? I'm running 25.5" rear tires, a rear out of an F-Body with 2.90's I believe, and I can go down the parkway doing 65-70MPH and only be turning around 2000RPM. Just driving down local hyways, doing 50-55ish I never take it out of third. I'm quessing a little cause I dont have the speedo hooked up yet, but does it sound like a standard 4-speed or maybe an overdrive unit?

I'm asking cause I want to get new rear gears to wake up the mildly worked 318 and I'm trying to decied a good ratio for performance without the car screaming when I'm doing 55-60. I was thinking 3.91's but I found a 4.10 unit, think that would be a bit much? Also I plan on running 27-28" rear tire in the near future.

Thanks

Butch