A little bit ago I posted about an inop Factory Temp Gauge on my 72 Dart. Turns out the sender was bad. Replaced it with a used one,and it "appears" as though it works.

I say "appears" because I can drive the vehicle and when I shut the car off,go into lets say, a convienence store and return,turn the key on,the gauge needle is moved up almost to center,but slowly returns to the first mark past the 120* mark once the engine is running and stays there the whole time im driving.To you all wouldnt this indicate its working properly?

Is it normal for a slant 6 to run that cool? Why might it run that cool and can it do any harm?

Very normal. When you shut off the engine the coolant remains still, and continues to soak up heat from the engine in the vicinity of the sending unit. When you restart the engine the flow cools it back down.

Is where it sits while driving(just above 120*)normal operating temperature? If so,man this thing runs cool.Shouldnt it be at least in the middle while driving?

No, I didn't catch the 120, thats way too cool. 160 is the low end of 'normal' operation. 120 won't even burn you (well it can, but it takes a few seconds)
Can you hold your hand on the rad after the engine is warmed up???

Havent tried it,but I did feel the upper radiator hose and its getting hot,so this indicates the T-stat is functional...right? Wait,could this low temp perhaps mean its not running a T-stat at all?

That says the T-stat is open. Doesn't tell you if its closing at cool temps. At 120 it should definately be closed.

Do you have an infrared thermometer?
or
You could intsert a regular thermometer in the rad while the engine warms up and see what happens. This will also allow you to look for no fluid movement during cold (closed stat) and movement when temp reaches something like 160-180 (stat opening)

That will tell you a lot about your stat and the accuracy of your gauge

Im waiting for my aftermarket Triple Gauge Cluster to come in the mail.Well see then just how accurate things are,It really doesnt bother me about the temp,I just thought it was odd.