Transmission questions. Manual valve body?

I realize this may be a dumb question but I'm a bit stumped. I'm new to auto transmissions, new to Mopar and new to my car. My car (72 Duster with 727 torque flight) was purchased from a dealer who provided zero info about the tranny. I've been driving the car for about a month, mostly in D mode and noticed that the transmission shifts from 1 to 2 to 3 rather early, meaning I'm in third by the time I hit around 25 mph. When I punch the gas, I don't get a downshift. If I shift in manual mode, the car shifts nice and hard. I have a center console stock shifter.

So I was looking online to see about adjusting the kick down linkage and lo and behold....I don't have one on my car. After a bit of reading, it seems like I *may* have a manual valve body but I honestly have no idea.

What advice can you experts give me? Should I have a kickdown assembly installed? Leave things alone and always shift in manual?